Stata Tutorial - Data and Statistical Services - Princeton University

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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
Page 5 and 6: 1.3. About Data for StataTo put a d
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Page 9 and 10: 2.3. Reading in an Excel fileIf the
Page 11 and 12: 2.4. Reading in Stata dataNow, let
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Page 24 and 25: Goal: run a multiple linear regress
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