Stata Tutorial - Data and Statistical Services - Princeton University

1.3. About Data for StataTo put a data set into Stata’s memory, the data set has to be in a format Stata understands. The following is a list of theextensions of files Stata can read directly.Data Format File Extension Command to read the dataStata .dta . useText (ASCII)Free or fixed columns .raw, .txt . infile usingComma separated values .csv . insheet usingFixed columns .dat . infix usingSAS export .xport, xpt . fdauseMS Access .mdb . odbcMany data download sites provide you with data already formatted for a common statistical program such as Stata, SPSS, orSAS. Formatted data often contain variable labels and value labels, that make it easier for you to understand the contents of thedata.If Stata data are not available, and you can choose a data format between SPSS and SAS, then I would recommend selectingSPSS. You can use SPSS to open SPSS data, then save the data as Stata data. SPSS versions 12 and up can save the data asStata 8 data 1 . Windows version of SPSS is available in McCosh 59 cluster or DSS computer lab. If data are only available inSAS format, you may use SAS to open SAS data, then create SAS export file, as Stata can read a SAS export file. Windowsversion of SAS is available in DSS computer lab. Unix version of SPSS and SAS are available at tombstone.Also, if you acquire data that are in a format other than Stata, you may use DBMS/Copy to convert them into Stata format.Windows version of DBMS/Copy is available in DSS computer lab. Unix version of DBMS/Copy is available at Tombstone.If you have SAS data, we recommend converting them into SAS transport file in SAS instead of using DBMS/Copy.DBMS/Copy has a known issue in converting value labels from SAS to Stata.If formatted data are not available, data distributers may provide set up files in Stata, SPSS, or SAS along with ASCII data.ASCII data set is a text file with rows (or columns) of numbers. If a set up file is available in Stata, you can attach the variableinformation using Stata. If a set up file is available in SPSS, it will be easier to use SPSS to attach the definition, then save thedata as Stata data. If a set up file is available in SAS, you may use SAS to attach the file definition, then create a SAS export filein SAS. You may also modify the set up files in text editors to use in Stata. Commands to define data are different in all threeprograms. If no set up files are available and only PDF codebooks are available, you will need to select the variables you wantto use and create your own set up file for Stata.If you need help in defining or converting data, please come by the Data and Statistical Services computer lab at A-16-H-3 inFirestone Library during walk-in hours or email data@princeton.edu. The hours and directions are available athttp://dss.princeton.edu. If you are emailing questions, please use your Princeton email. Our resources and assistance areavailable to Princeton University community members.Stata data format has changed from version 9 to version 10. Stata 10 can read data saved for Stata 9, but Stata 9 can not readdata saved for Stata 10, while both has the same extension .dta. If you plan to use Stata 9 after using Stata 10, you may save thedata as Stata 9 data in Stata 10. Followinig commands allow you to save data as Stata 9 data in Stata 10.CMD: . saveold filenameMNU: File=> Save As. Then select “Stata 9 Data” from the drop down list for box “Save As Type:”1 Stata 8 and Stata 9 data are interchangeable.Page 5 of 28

2. Read in data2.1. Reading in an ASCII data file using a Stata set up file.Often times, you may obtain a command and a dictionary files as a set of Stata set up files along with a data file. I suggest thatyou save all three files in the same directory. The command file has an extension .do, the dictionary file .dct and data file .txt(or .dat). The command files in Stata are also called do files. Sometimes the do file contains the dictionary, and you have twofiles, do file and data file. The procedure is similar to having three files.As an example, I downloaded a Stata set up file and data file for National Health Interview Survey from the Inter-universityConsortium for Political and Social Research (ICPSR) web site, http://www.icpsr.umich.edu. The files usually are zipped whenyou download. I extracted the zipped files using WinZip, and put them in C:\StataHandsOn\SampleData directory. WinZip isavailable in DSS lab computers. OIT computers do not have WinZip, but extraction software that comes with Windows canunzip files.Then I opened the Stata command file using NotePad (any text editor will do, but not a word processor like MS Word).Instructions are given at the beginning of the command file, sandwiched between lines of asterisks as in the picture below. Aforward slash and an asterisc (/* texts */) makes the texts in between comments. Follow the instruction and specify the nameand the path of the data, dictionary, and output data files in the do file. Here is a copy of the beginning of the do file for theNational Health Interview Survey data.Page 6 of 28

**************************************************************************| STATA SETUP FILE FOR ICPSR 04349| NATIONAL HEALTH INTERVIEW SURVEY, 2004| (DATASET 0004: SAMPLE ADULT)|| Please edit this file as instructed below.| To execute, start Stata, change to the directory containing:| - this do file| - the ASCII data file| - the dictionary file|| Then execute the do file (e.g., do 04349-0004-statasetup.do)**************************************************************************/set mem 40m /* Allocating 40 megabyte(s) of RAM for Stata SE to read thedata file into memory. */set more off /* This prevents the Stata output viewer from pausing theprocess*//****************************************************Section 1: File SpecificationsThis section assigns local macros to the necessary files.Please edit:"data-filename" ==> The name of data file downloaded from ICPSR"dictionary-filename" ==> The name of the dictionary file downloaded."stata-datafile" ==> The name you wish to call your Stata data file.Note: We assume that the raw data, dictionary, and setup (this do file) allreside in the same directory (or folder). If that is not the caseyou will need to include paths as well as filenames in the macros.********************************************************/File PathFile NameReplace thefile nameshere.local raw_data "C:\StataHandsOn\SampleData\04349-0004-Data.txt"local dict "C:\StataHandsOn\SampleData\04349-0004-Stata_dictionary.dct"local outfile "C:\StataHandsOn\SampleData\health.dta"/********************************************************Section 2: Infile CommandThis section reads the raw data into Stata format. If Section 1 was definedproperly, there should be no reason to modify this section. These macrosshould inflate automatically.**********************************************************/infile using `dict', using (`raw_data') clearOnce you have the file paths and names inserted into the do file, execute the do file (in this example named 04349-0004-Setup.do) in Stata by giving a command:. do 04349-0004-SetupIn this case, you do not need to modify the dictionary file. In some cases, you may need to specify the data file path and namein the dictionary file.I specified in the do file to name output Stata data as health.dta (see the third line that starts with “local outfile”), and you seethe file listed in the directory in the picture on the previous page.You may obtain a data definition file for SAS or SPSS. The idea of attaching the data definition in SAS or in SPSS is the sameas in Stata, except that their data definition would only be in one file, and they need to be executed in respective program.Please refer to separate handouts for details in running data definition files using SAS or SPSS.Page 7 of 28

2.2. Creating a Stata set up file.When you have an ASCII data file but not a set up file, you will need to create one to define variables. An ASCII data filecontains many rows of numbers and Stata will not know which numbers belong to which variables. You also need to define thetype of variables, whether they are numeric (numbers) or string (texts or characters). ASCII data may be in free format, commaseparated, or fixed columns. Example files used in this exercise are 2008 Democratic and Republican PresidentialPrimaries/Iraq (United States), downloaded from the Roper Center, http://www.ropercenter.uconn.edu.Here is a portion of a fixed column ASCII data, called lat544.dat:.25 1 2391 1 1 1 4 2 1 2 3 3 2 & 338 & & & & & & & 131 5 6 7 7 3 1 4 4 4 4 4 4 & & & & 0 & & & 2 0 2 2 1 2 2 0 1 2 1 1 2 2 2 1 4 2 & 5 1 2 3 2 65 & 6 112 5 1 21.66 2 9041 1 1 1 4 1 2 1 2 3 2 & 8 13& & & & & & & 5 2 1 3 3 6 3 1 4 & & & & & 4 4 4 4 4 3 5 & 1 2 1 2 4 2 2 0 1 2 1 4 1 1 1 1 5 2 & 1 3 4 1 1 55 & 6 6 1 5 1 2.47 3 2122 4 4 4 2 3 2 4 2 1 1 & 1 2 112 5 3 6 6 6 & & & & & & & & 1 & & & & & 1 1 1 1 0 & & & 2 2 2 1 1 1 1 4 2 1 2 1 2 1 1 1 1 1 & 1 7 & & & 65 & 7 8 1 2 1 1.41 4 4122 4 4 4 1 2 4 1 1 1 1 & 1 8 7 2 3 1 2 5 6 & & & & & & & & 1 & & & & & 1 1 1 1 0 & & & 3 3 3 3 1 1 1 4 0 1 2 0 2 1 2 1 2 1 & 1 2 3 1 1 & 5 5 5 2 2 1 1A portion of the corresponding codebook says ...Data LocationsVariable Rec Start End FormatWTVAR 1 1 7 F7.2CASE 1 8 14 F7.0AREACODE 1 15 21 F7.0TRACK 1 22 23 A2GWBUSHJO 1 24 25 A2GWBECON 1 26 27 A2GWBIRAQ 28 29 A2This means that the variable WTVAR is at the first record, startsfrom the column 1 and ends in column 7. The data format is F7.2,meaning that it’s a numeric variable with width 7, and includes twodecimal places. In the data above, .25, 1.66, .47, and .41 correspondto this variable.To define the variables in Stata, you can create a “dictionary” file thatcontains the variable information as in below. You can use any texteditors, but here we use Stata’s “do-file editor.” Open a do-flie editor:CMD: . doeditMNU: Window-> Do File Editor -> New Do File.There, typeinfix dictionary using H:\lat544.dat {WTVAR 1-7CASE 8-14AREACODE 15-21str TRACK 22-23str GWBUSHJO 24-25str GWBECON 26-27str GWBIRAQ 28-29}Carriage return is Stata’s default signal to endcommands. So it is important to type as it appearshere. The first line ends after the squiggly-brace({ ), each variable name and the column locationsis in one line, and the last squiggly-brace ( }) is onits own line.and save the file as a dictionary (.dct) file in the same directory as the data file. For example, I saved the file as H:\ lat544.dct, asI have the data file, lat544.dat, at the root of H drive. The str in front of variable names indicates that they are string variables.I have omitted record number, as there is only one record in this data. If your data file has more than one record, you need todefine which record you are referring to for each of the variables. Please see help infix to see the syntax for multiple records.Once you save the dictionary file,CMD: . infix using H:\lat544.dctMNU: File => Import => ASCII data in fixed format. Then Browse to find the dictionary file name and path.Stata will show the following in the output window.. infix dictionary using H:\lat544.dat {WTVAR 1-7CASE 8-14AREACODE 15-21str TRACK 22-23str GWBUSHJO 24-25str GWBECON 26-27str GWBIRAQ 28-29}(1373 observations read)Check with the codebook and see if the total number of records is 1373.Page 8 of 28

2.3. Reading in an Excel fileIf the data file is “clean,” all you need to do is to save the file as .csv file in Excel and import it into Stata. However, if the datafile is “not clean,” editing it may make it easy to import it into Stata. Here is an example of a “not clean” excel file.header lineseach value includes aspecial character anda commathe second line.variable names include specialcharacters, starts with a number,or have spaces between wordsblank lineand columnStata reads the values in the first lineas the variable names. Header linesprevent the program to read thevariable names. Also, the programexpects data from the second line, soin this example, Stata will convert allthe variables as string.The variable names in Stata can nothave special symbols or start with anunderscore (_).The following is an example of a“clean” excel sheet. It has thefollowing characteristics:• The first line has Statavariable names: 32characters or less, no specialcharacters, and not startingwith an underscore ornumber. Data begin from• No blank rows or columns. (Blankcells are ok. Stata automatically adds aperiod (.) if numeric. Do not manuallyadd . in blank cells.)• Missing numeric data should be anempty cell or values defined as missing,such as 0, 9, or 99. A space (storedspace, different from empty), dot, orany other non-numeric character suchas n/a will cause the variable tobecome string.• Commas in numbers or texts areparticularly problematic because Statamay see them as a delimiter and willnot read the data properly. Youshould remove the commas fromnumeric values before saving the file.Page 9 of 28

Once you examine the file and make sure that the file is clean, here is a step-by-step instruction for saving a worksheet as acomma separated values file in Excel. As a practice, let’s read in a sample excel data.1. Open the Internet Explorer and downloadauto.xls fromhttps://webshare.princeton.edu/users/furuichi/auto.xls and select Save to Disk.2. Save the file in your H:\ directory.3. Start Excel and read the file by selecting File=> Open4. Under the File menu, select Save As, thenSave as type 'CSV' (comma separated values).5. Open Stata6. Change the directory in StataNote: Renaming the file with a .csv extension in theWindows Explorer is not the same as saving the file asa .csv file.If the spreadsheet is small, you may copy the data and paste them intoStata’s data editor. Highlight all data in Excel, and select Edit =>Copy. Open Stata, then select Data Editor. Right click and selectPaste, or press Control and v keys at the same time to paste the Exceldata contents into Stata’s Data editor.Stata may mistakenly read numeric variables as strings. Check thatoriginal numeric values are numeric in converted Stata data by issuingthe command –describe- in Stata and examining the storage type. If thevariable has storage type that starts with "str," then Stata has made it a string variable.If you see that a numeric variable in the original data file hasstring storage type in Stata, go back to Excel, and change thevariable’s format into numeric, and re-save the file as .csv file.Here is how:1. Highlight the column with the numeric variable name.2. Click Format => Cells.3. In the Format Cells window, select Number tab.4. Under Category drop down list, select Number.5. Click OK6. Under the File menu, select Save As, then Save astype 'CSV' (comma separated values).in Stata. At the command prompt, type. destring stringvariablename, replaceYou can also change the variable type from string to numericFor this command to work, the stringvariablename can not have any non-numeric characters as its value. If it fails, check thevalues of the variable to find non-numeric characters.Page 10 of 28

2.4. Reading in Stata dataNow, let’s start using Stata. From an OIT computer, a link to Stata may be found in Start => All Programs => Stata10 =>StataSE10. A shortcut to Stata may be available from the Special Applications folder on the desktop. Double click the Stataicon.Typing commands in the Command windowStata starts in its default working folder, typically C:\ProgramFiles\Stata\Stata10. Let’s change the directory toH:\StataHandsOn.. cd H:\Let’s create a StataHandsOn directory.. mkdir StataHandsOn. cd StataHandsOnBefore reading in a data file, let’s open a log file. A log file storesyour output that appears in the Results Window.. log using auto1.logNow let’s read in 1978 Auto data. It is a data file that comes withStata installation, and available in Stata format.. sysuse autoSuppose you want to add a label to the data, so that you canremember what the dataset is about. It is convenient if you makemany subsets of data files from the original file. As an exercise, let’slabel the data that it is for 1978 auto data for hands on training.. label data “1978 auto data for hands on training”We will work more on this data, but let’s save the data at this time.We’ll give it a name, testauto.. save testautoLet’s close a log file at this time and look at the file. Issue acommand:. log closeUse a text editor or MS Word to open the log file.You can also view a log file in Stata. Remember to include theextension with the file name when typing the –view- command.. view stata1.logLet’s clear the data in the memory at this time and exit from Stata.. clear. exitusing MenusFile =>ChangeWorkingDirectory..(navigate toH:) thenselect MakeNew FolderFile => Log=> Begin....File=>Exampledatasets =>Exampledatasetsinstalled withStata =>(auto.dta) useData=>Labels=>Label datasetFile=> SaveasFile=> Log=>CloseFile=> Log=>ViewFile=> Exitusing MenusNotes and TipsIf you don't change the directory, Stata will assumethat the file name you type is in the defaultdirectory. The log and data files you save will alsobe in this directory unless you change it.If you do not open a log file at the beginning of thesession, the output will only be available in thetemporary memory. Once you exit from theprogram, the output will be lost.A log file with the extension ".log" is a plain text file.This means you can open and read it in almost anytext editor or word processor. If you issued the –log- command without the file extension,. log using stata1Stata would create "stata1.smcl." smcl is a log filetype specific for Stata.Notice that “log on (text)” appear on the rectangularspace between Results Window and CommandWindow once you begin a log.If you issue –save, replace- command withoutspecifying a file name, what is currently in memorywill overwrite original input file. To avoid losingoriginal data file by mistake, always remember tomake a master copy before starting to work ondata.Notice that “log on (text)” disappeared from therectangular space after closing the log.Results window can not be cleared while insession.Page 11 of 28

Typing commands in the Command windowRestart Stata, and check if we are in the directory we first specified.. pwdthen list the files:. lsIt should show that we are at H:\StataHandsOn directory. Can youfind the testauto.dta and auto1.log? What is the size of the data file?It is a small data file, of only 5.4 kilobytes. As you may have seenat the very first screen, Stata’s memory may be initially set to 10megabytes. Because this data file is smaller than what Stata allowsin the memory at this time, you will not have problems reading inthe data.Let’s open the log file back on to continue to save the output on thesame log file. Issue a command:. log using stata1.log, appendTo read testauto data:. use testautoIf the data file is larger than Stata’s current memory, it will issue anerror message. Check the file size and set memory to give Statamore space. For example, if the data file is 36 megabytes, type. set memory 40mIt gives 40 megabytes worth of data memory in Stata to read thedata.File=>Changeworkingdirectory...File=> Log=>Begin, thenselectstata1.log,and Appendto existing fileFile=> Open(no equivalentmenu)Notes and Tipspwd stands for Present Working Directory.You can see the directory Stata is pointing at bylooking at the bottom bar of Stata’s window. If youare not at H:\StataHandsOn, change the directoyrby typing in the Command window:. cd H:\StataHandsOnYou can, of course, start a new log file instead ofappending the new results to the existing log file.To start a new log file, give a new file name as in:. log using stata2.logYou could clear the data in memory and read in anew data file in one step, by issuing a command:. use testauto, clearOnly one data file can be read into the Stata’smemory at a time. You need to clear the memorybefore reading in another set of data. (You can,however, open many instances of Stata in onecomputer.)To see the maximum limits in Stata, type in theCommand window:. help limitsReview Questions:1. How can I start Stata?2. Which directory is this program pointing at?3. How can I change the directory to H:\?4. How large is the Auto data?5. How do I read the data into Stata?6. How do I label the data?7. How do I save the data?8. I don’t know what commands to use. How do I get a helpin Stata?9. How do I record the output?Hints:. pwd. cd C:\mydata. dir . ls. set memory 20m. use filename. label data “descriptions”. save filename, replace. help commandname. search keywordPage 12 of 28

Typing commands in the Command windowThe –list- command is particularly helpful to use after sorting data,or combining with if. For example, you can obtain five minimumvalues of MPG by listing the first five records after sorting.. sort mpg. list mpg in 1/5Suppose you want to see the make of the cars whose price is lessthan $5000. Try:. list make price if price greater than>= greater than or equal to< less than= b ora>=b), but you cannot put spaces within them (e.g., it must be‘>=’, not ‘> =’).Combining tests: -and- and –or--if- can be combined with and (&) to evaluate for more than oneconditions. Let's say you want to find out the MAKE of the carswhose MPG is greater than 30 and PRICE is less than $5000.. list make if mpg>30 & price30 | price

Typing commands in the Command windowNotes and TipsAbout Missing ValuesStata indicates a missing numerical value as a period (.), and amissing string value an empty string, “”. Missing numerical valuesare larger than numerical numbers.We know from the previous examination (.codebook rep78) thatfive out of 74 records of REP78 are missing. We can use the periodto indicate missing record in the command and see which MAKEof the cars are missing in the data.. list make if rep78 >= .Check that a period is the largest values in rep78, by sorting byrep78 and listing the last six values.. sort rep78. list make rep78 in -6/-1Data=>Describe data=>List dataclick by/if/in tabin list dialog boxData=> Sort=>Ascending sortReview Questions:5. How many variables and records are in the data?6. What does the note say?7. How can I add notes or comments to the data?8. What variables are in the data?9. How do I sort?10. Which variables have missing values?11. List the cars for which data is missing.12. List the cars whose repair record is less than 3 and theprice is less than $5,000Hints:. describe. codebook . labelbook. inspect. summarize. sort . gsort. list [if] [in]Page 18 of 28

5. Obtain descriptive statisticsGoal: find out number of missing records, minimum and maximum values, means, and medians, viewfrequency tables, and cross tabulations.The commands that are useful for getting basic descriptive statistics include tabulate, summarize, tabstat, and table.Typing commands in the Command windowThe –tabulate- command gives you a frequency distributionif only one variable is specified, and a cross-tabulation if twovariables are specified. If two variables are specified, the firstvariable will be shown in rows, and the second in columns.. tabulate rep78. tabulate rep78 foreign-summarize- gives the number of valid observations, mean,standard deviation, minimum, and maximum values.. summarize price mpgWhat if you wanted to see the average MPG for foreign anddomestic cars? The –tabulate- command can be combinedwith –summarize- to produce a summary of one variable forthe variable specified in –tabulate-. For example, if you wantto see the average MPG by car type, type:. tabulate foreign, sum(mpg)If you want to see more statistics such as total, range, ormedian, you may use tabstat.. tabstat price mpg, stat(sum, range, median)There are more statistics you can see using tabstat. See . helptabstat for a list of statistis.The –table- command lets you create three-way (or four-wayif combined with –by-) cross-tabulations. We can try thatafter we create more categorical variables in the next section.using MenusStatistics=>Summaries,tables, andtests=>Tables=> Onewaytables, or Allpossible twowaytabulationsStatistics=>Summaries,tables, andtests=>Summaryand descriptivestatistics=>SummarystatisticsStatistics=>Summaries,tables, andtests=>Tables=>One/two waytable ofsummarystatisticsNotes and Tips–tabulate- can not cross-tabulate more than twovariables. If you have more than two categoricalvariables to crosstab, use –table- (see below).Because –tabulate- gives you frequency counts, itmakes sense to use it for categorical variables thancontinuous variables.It would make sense to summarize continuousvariables rather than categorical variables.You can also see the average MPG by FOREIGNby using –by- and –summarize-.. bysort foreign, summarize(mpg)To use –by-, the data have to be sorted byFOREIGN. You could do .sort foreign, then .byforeign, sum(mpg). . bysort foreign does thesorting and by in one step.Stata allows shorthand in some commands. –sumisthe shorthand for –summary-. The shorthand isshown as an underscored letters in the help page.Review Questions:1. Which five cars yield the lowest gas mileage?Which five cars yield the highest gas mileage?2. What is the average price and average miles pergallon (MPG) of a car in the 1978 auto data?3. What is the average price of cars that are below andabove the mean MPG?4. What is the median MPG?5. How are price and MPG different for domestic andforeign cars?6. How can I see the number of cars by the car type?7. How are the cars distributed by the repair records?8. Compare frequency-of-repair records for domesticand foreign cars.Hints:. sort. list. summarize. tabulate. table. by groupingvarname: summarize varnamesPage 19 of 28

6. Transform variables and recordsGoal: create and label new variables, modify existing variables, keep or delete variables and records from the file,recode values, create dummy variables from existing variables.The basic commands for creating new variables and modifying old ones are –generate- and –replace-.Typing commands in the Command windowThe command. generate newvar = somethingcreates a new variable named newvar and sets it equal tosomething. Something can be a number, a string, a mathematicalexpression, or a function of other variables. You can combine–if-, -&-, and -|- in generating new variables.. generate two = 1+1. generate mycars = 1 if (rep78==1 & priceCreate new variable,then click if/in tab,select Create..., typein criteria in thewindow, click OKData=> Create orchange variables=>Change contents ofvariableData=> Variableutilities=> Keep ordrop variables=>select Drop variablesData=> Create orchange variables=>Other variabletransformationcommands=>Recode categoricalvariableNotes and TipsYou normally want to use replace forsecond and later steps in multi-stepvariable creations. When you modifyexisting variables, make sure you will stillhave a way to recreate the originalvariable or have a back-up copy of thevariable. Once you write over existingvariable, there is no way to get theoriginal data back.. list hirep if rep78==.Notice that hirep disappears from thevariables window. Once you delete avariable, you can not undo the deletion.If you issue a command –preservebeforeremoving a variable, you mayrestore deleted variable by issuing acommand –restore-. This is a temporarymeasure and only works as a set. Onceyou issue –restore- command, you needto issue another preserve command torestore.If you do not specify a new variable namewith the generate option, you willoverwrite the original variable. Let’s trythat with –preserve- and –restorecommands.. tab rep78. preserve. recode rep78 (1/2=1) (3/4=2)(5=3). tab rep78. restore. tab rep78gen is a short for generate.Page 20 of 28

Typing commands in the Command windowWe have already seen how to create a dummy variable (whoseoutcome is either 0 or 1) using –generate- and –replace-.Another easy way to create dummy variables is to use –tabulate- command. The –tabulate- command, when used witha generate option, produces dummy variables for each value.For example, suppose we want to create a dummy variable foreach of the outcomes of the categorical variable REP78.. tabulate rep78, gen(dumrep78)Suppose you want to group a continuous variable, PRICE, intofive equal ranges. First find out the minimum and maximumvalue that you want to use to group the PRICE by using –summarize-. Then,. generate ivprice = autocode(price,5,3291,15906)If you want to group PRICE into five groups of equalfrequencies, first sort PRICE, then issue the followingcommand:. sort price. generate fqprice = group(5)Now, we have several more categorical variables to make a fourway table. Let’s create a table of repair records by HIREP byIVPRICE by FOREIGN. Here is how:. table rep78 hirep ivprice, by(foreign)You can label the variables so that you know what they are lateron. Let’s add a label to HIREP as an example.. label variable hirep “repair record is 3 or higher”. label define yesno 1 “yes” 0 “no”. lable values hirep yesnousing MenusData=> Create orchange variables=>Other variablecreationcommands=> Createindicator variablesData=> Create orchange variables=>Create new variable,then enter autocodefunction in the boxData=> Sort=>Ascending sortStatistics=>Summaries, tables,and tests=>Tables=> Table ofsummarystatistics(table)Data=> Labels=>Label variableData=> Labels=>Label values=>Define or modifyvalue labelsData=> Labels=>Label variableData=> Labels=>Label values=>Assign value labelsto variableNotes and TipsScroll down the Variables Window to seewhat Stata created. Alternatively, viewthe list of variables by:. describeYou can also add notes to the variables.. note hirep: “temporary variable createdon October 1, 2006”When you describe data, (-describe-) youwill see an asterisk (*) by the variablelabel indicating that the variable hirep hasnotes.See the notes by typing. notesThe maximum number of variables youcan list in –table- is three.-label variable- adds a label to thevariable.-label define- defines values of a lable.The label name can be different from thevariable name, and can be used for othervariables.-label values- attach label to the variable.Review Questions:What is the command to1. create new variables?2. delete variables?3. regroup variables?4. group continuous variables?5. create dummy variables?Hints:. generate newvar =. drop varnames. recode oldvar (1/2=1) (3/4=2) (5=3),gen(newvar). generate varname = group(5). generate newvar =autocode(oldvar,5,min,max). generate newvar = 0. replace newvar = 1 if oldvar > 6165Page 21 of 28

7. GraphGoal: view the relationships of the variables by graphing and save graphs.Stata has several graphs for graphing distributions of individual variables, the relationship of the variables, as well as many morespecialized graphs. Shown here are commands for some basic graphs. You may explore graphs using the menus as well. InStata, graphs appear in separate windows that pop up. The graphs do not appear on the Results window, and will not be storedin the log file. If you want to save the graphs, you will need to save each graph as a file.Typing commands in the Command windowHere's a simple histogram of PRICE.. histogram priceYou can see the histogram separately for different groups.For example, you can see a histogram of price for foreignand domestic cars separately and have Y values infrequency.. histogram price, by(foreign) freqAnother popular graph is box plot. Let’s see box plots ofprice by foreign.. graph box price, by(foreign)The basic command for drawing a bivariate graph is twoway.The command twoway is followed by a keyword indicatingthe type of graph. To obtain a scatter plot showing therelationship between MPG and WEIGHT, type. graph twoway scatter mpg weightWe can obtain the scatter plot by the car type, FOREIGN .. graph twoway scatter mpg weight, by(foreign)Twoway graphs can be overlaid: you can draw two twowaygraphs on the same set of axes. A common use of this is todraw a scatterplot with a regression line laid overtop of it toshow how the regression line fits the data.We will overlay scatter plot of with regression line fit forMPG and WEIGHT.. graph twoway (scatter mpg weight) (lfit mpg weight)Let’s save the graph. On the Command Window, type:. graph save OverlaidMpgWeightOnce it’s saved, close the graph window, and bring it upagain.. graph use OverlaidMpgWeightReview Questions:How can I …1. make a histogram of MPG?2. see a scatter plot of MPG against WEIGHT?3. fit a regression line over the previous scatter plot?4. bring the graph up again after I close the graphwindow?using MenusGraphics=> Histogram,insert variable name PRICEin the Variable: box andcheck the box next to Bins,change the number to 5Graphics=> Box plotGraphics=> TwowayGraph, click Create, selectScatter in the Basic plots:box, Y variable: mpg, Xvariable: weight, clickAccept, then in the “By” tab,select Draw subgraphs...,input foreign in Variables:boxGraphics=> TwowayGraph, click Create, selectFit plots under plotcategory, and Linearprediction under Fit plots:,Y variable: mpg, X variable:weightFile=> Save Graph...orIn the Stata Graph window,File=> SaveFile=> Open Graph...Notes and TipsFor an introduction to Stata graphs,type. help graph introDefault Y value of histogram is density.To see the histogram in frequency orpercentage, type freq or percent aftera comma:. histogram price, freqTo see more options, see.help histogramTyping scatter y x draws a graph of yagainst x.Here, scatter and lfit are plot typeswithin the twoway family. Alternatively,you can use || to separate the plottypes.. graph twoway || scatter mpg weight ||lfit mpg weightYou do need to separate the plot typesby the parentheses or the pipes.Hints:. histogram. graph twoway scatter. graph twoway (scatter y x) (lfit y x). graph save. graph usePage 22 of 28

Goal: run a multiple linear regression model.9. Obtain linear regression estimatesTyping commands in the Command windowIn estimating relationships among variables, you mayfirst want to examine how the variables are correlated.We suspect that MPG and WEIGHT are correlated.Let’s see the correlation:. correlate mpg weightIn addition, we suspect that the correlation may bedifferent between foreign and domestic cars. We cancombine the –correlate- command with a by statement.Before using a by statement, the data need to be sortedby the by-variable.. sort foreign. by foreign: correlate mpg weightIt seems that mpg and weight have a relatively highcorrelation. The correlation is different for foreign anddomestic cars, so foreign must also impact MPG.From the scatterplots we saw earlier, we also discoveredthat the relationship between WEIGHT and MPG isnot exactly linear. We’ll include a square of WEIGHTto improve the model. Let’s run a regression estimatingMPG by WEIGHT, WEIGHT2 and FOREIGN.. regress mpg weight weight2 foreignAfter estimating a regression model, we can use thevalues estimated by the model, called post-estimationvalues. Using estimated MPG, we can see how theestimated line fit the original distribution by viewingoverlaid graph. To do so, we first need to create avariable for the predicted MPG. We’ll call thisMPGHAT.. predict mpghat. graph twoway (scatter mpg weight) (line mpghatweight), by (foreign)Review Questions:1. What is the correlation between MPG andWEIGHT?2. Is the correlation different between domesticand foreign cars?3. How do I obtain regression estimates?4. How can I compare observed and predictedvalues on a graph?using MenusStatistics=> Summaries, tables,and tests=> Summary anddescriptive statistics=>Correlations and covariancesData=> Sort=> Ascending sortStatistics=> Summaries, tables,and tests=> Summary anddescriptive statistics=>Correlations and covariances, in“by/if/in” tab click Repeatcommand by groups, insertforeign in Variables that definegroups:Statistics=> Linear models andrelated=> Linear regressionStatistics=> Postestimation=>Predictions, residuals, etc.,Graphics=> Two-way graph(if there are already definedplots in “Plot definitions:”, eitherDisable or Edit them to createnew combinations)Notes and Tips. pwcorr mpg weight, star(.05)adds an asterisc (*) next to thecorrelation coefficients that arestatistically significat at 95% level.You can also sort and use “bystatement” in one step:. bysort foreign: correlate mpg weightThere are series of regressiondiagnostics you can do using graphs.See UCLA’s Stata tutorial site formore information.To compute a square of WEIGHT,WEIGHT2, you can multiply WEIGHTby itself, or raise it to the power of 2.. generate weight2 = weight*weight. generate weight2 = weight^2do the same thing.Stata has a series of “post estimationcommands.” After running aregression estimates, for example,you can test if the coefficients arestatistically significantly different from0, or from another independentvariable (wald test), or test forheteroscedasticity. For details, see. help regress postestimation,xb that appear as an option whenmenu is used is a default incommand window input. It will notappear in the Results window whencommand is input in the Commandwindow.Hints:. correlate. by varname: correlate. regress. predict yhat. graph two way (scatter y x) (lineyhat x)Page 24 of 28

10. Do filesWhen you have rather intense computations or repeat/modify existing computations, it may be helpful for you tocreate a file that contains a set of Stata commands. Such files are called “do files” in Stata. Do files can be createdby manually entering commands in any text editors, or using Stata’s do-file editor. In Stata, do-file editors can beinvoked by:CMD: .doeditMNU: Window=> Do-file editor=> New do-fileYou may also create do-files by saving commands you submit interactively. When you start a Stata session, start“command log,” which is a log file with only the commands. It by default attaches .txt file extension if you do notspecify the extension. If that is the case, you can change it in Window’s file explorer. For this command, I have notfound a menu version.CMD: .cmdlog using filename.doIf you forget to start a command log, you may save the commands in the Review window. First, right click in theReview window then, select “Select All”. Right click in the Review window again, then select “Send to Do-fileEditor”. You can eliminate error commands by clicking the _rc on top of the Review window, which sorts thecommands by the errors, then select the error commands, right click, then “Delete”. You can resort the commandsin the original order by clicking the top of the numbered column on the far left. For the same token, you can sortthe commands by clicking the top bar where it says “Command” and delete commands like –browse- and –help-.By the way, if you use menu for help and search, they do not appear on the Review or Results window.11. Shortcut menusOpen dofileeditorOpen dataeditorOpen databrowserOpen dataSave dataQuit 4Print resultsLog 1 Open/ closeviewersGraphwindow 2ScrollResultswindow 31. Begins log if no log file is open. If a log file is open, it lets you view, close, or suspend the log. You may appendto the previous log by selecting an existing log file. Dialog box menu changes accordingly.2. Moves graph window upfront. It only becomes active when a graph window is open.3. Scrolls the Results window one screen at a time, when you have –more- at the bottom of the Results window. Itis equivalent to hitting the space bar or clicking –more-4. Quit processing. Useful when a process is taking a log time and you want to stop the process, or when you have–more- but do not want to see more. It is equivalent to hitting q in Command window or Ctrl-c at the same time.Page 25 of 28

12. Exporting resultsYou can copy what appears in Results window by highlighting and right clicking. There are several options: Copy Text, CopyTable, Copy Table as HTML, and Copy as Picture. Here are pasted tables for each.Copy TextRepair |Record 1978 | Freq. Percent Cum.------------+-----------------------------------1 |2 |282.9011.592.9014.493 | 30 43.48 57.974 |5 |181126.0915.9484.06100.00------------+-----------------------------------Total | 69 100.00Copy TableRepairRecord 1978 Freq. Percent1 2 2.90 2.902383011.59 14.4943.48 57.974 18 26.09 84.065 11 15.94 100.00Total 69 100.00Copy as PictureCum.If you are pasting tables into Excel, copyingeither as table or HTML will work well.If you are pasting tables into Word, copyingas picture seems to produce the bestapperance. If you save them as picture,though, modifying the contents can only bedone using a graphic software.Copy Table as HTMLRepairRecord 1978Freq. Percent Cum.1 2 2.90 2.902 8 11.59 14.493 30 43.48 57.974 18 26.09 84.065 11 15.94 100.00RepairRecord 1978 Freq. Percent Cum.1 2 2.90 2.902 8 11.59 14.493 30 43.48 57.974 18 26.09 84.065 11 15.94 100.00Total 69 100.00Total 69 100.00Log files with extension .log can be opened in Word. Log files with extension .smcl will show the tags for Stata. See thecommand in the next section to convert .smcl files into .log files.Graphs saved as a picture (see section 7. Graph) can be imported into a document. There are several options for the format.Use the drop down list in Save As box for the selection. Graphs can also be copied and pasted into another application like MSWord. Right click the graph you want to copy, then select Copy Graph. Paste the graph in Word using Edit=> Paste, rightclick and Paste, or hit Control and v at the same time. When the graphs are copied into Word 2003, they may not appearcorrectly when the file is converted into Word 2007.There are also user created commands to output results. You may check out commands such as outreg, outreg2, estout, tabout,est2tex, mktab, and xml_tab. To read about the commands, use search. For example, type in Stata’s command window,. search outreg, allNote about user created commands: Stata, being a programmer friendly program, makes it easy to install and use user madecommands. If you see a user made command that you want to use, you can install it by first finding the command by searchingfor it (you can also type -findit- commandname in Stata’s Command window) and clicking the blue letters “click here to install.”The help pages on the commands become available after installing the program.Page 26 of 28

13. Other helpful commandsIf working with a large file:You can describe data without loading the data by specifying the location and the name of data file.. describe using datafilenameYou can load only the variables you need by specifying the variable names.. use var1 var2 var3 using datafilenameSome commands produce a log that is more than a page long (-compress-, for example). To save yourself frompressing a key to scroll each page, you may use. set more offIf you are seeing –more- at the end of the screen after typing search, and want to quit seeing more screens, pressq or control and c keys at the same time. Clicking red X button does the same thing.You can save some memory by compressing the data.. compressShortcutsStata can fill in a variable name with a tab key aftrer enough characters to recognize the name are entered. Forexample, while you have the auto data open, try:. describe h [hit tab key] Stata fills in the rest of the variable name as headroomYou can bring up previously used commands in the Command window by hitting Page Up key.You can refer to a set of variables with the same stem using an asterisc (*), as in:. describe weight* if you had created weight2, it will show both weight and weight2MiscellaneousIf you forget to start a log file at the beginning of a Stata session, but want to save what you have in the outputwindow, use. translate @Results outputfilename.txtThe file can be viewed using a text editor or a word processor.Note: -translate- only saves what is in the buffer (what you see in the Results window). Depending on the length ofthe output you had produced, earlier results may have been lost. It is a good habit to start a log file each time youstart a Stata session.If you created Stata log file that has a file extension .smcl, you can reformat it into a text file by giving the command:. translate filename.smcl filename.logIf you want to perform a mathematical operation on the spot, you can use the –display- command.. display 1+1 => will return 2Page 27 of 28

14. On-line tutorialsUCLAhttp://www.ats.ucla.edu/stat/stata/UNChttp://www.cpc.unc.edu/services/computer/presentations/statatutorial/Princetonhttp://data.princeton.edu/stata/http://www.princeton.edu/~eszter/stata.htmlhttp://www.princeton.edu/~otorres/Stata/http://opr.princeton.edu/computing/software/stata/intro/default.asp15. ReferencesHamilton, Lawrence C. 2006. Statistics With Stata. Updated for Version 9. Pacific Grove, CA: Duxbury Press.Stata Corporation. 2008. Using Stata Effectively: Data Management, Analysis, and Graphics Fundamentals.Data and Statistical Services, Princeton University. Fall 2007. Stata Hands-on Instruction Guide. Windows version9.0.Page 28 of 28