STATA 11 for Windows SAMPLE SESSION - Food Security Group ...

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Stata 11 Sample Session Section 2 – Restructuring Data Files – Table Lookup & Aggregation Create a new file which is a household level file rather than a household-product level file The collapse command The “|” is a symbol for the word OR. We are telling Stata to select all cases with prod equal to 47 or prod equal 30 or prod equal 31 and so on ... 4. Click on the copy button, switch to the do-file editor, paste the command, switch back to the dialog box and click on Ok 5. Add a comment to explain what you have done. The Stata command is: keep if prod==5 | prod==6 | prod==30 | prod==31 | prod==41 | prod==44 | prod==47 Only cases with these product codes will now be used for analysis. Note that 464 observations were dropped. You can use the tabulate command to verify that you now have only 7 crops in the file. In the Command window you can easily type tab prod There should be 1,239 cases remaining for the 7 crops that are considered staple foods. Now, we need to know how many calories were produced per household for all these 7 staple food products combined. To do this, we need to sum, for each household, the values of cprod_tt for all of the food crops the household produced. In other words, we need to create a new household-level file from the current household-product level file where there is only one case per household. Stata uses the command “collapse” to aggregate the number of cases at one level to a new level. We will sum all the cases for each household to create just one case for household. To create the new household-level file, we use the command: collapse. Stata always uses the working data file as the file to be collapsed. 1. From the Data menu select Create or change variables then select Other variable transformation commands then select Make dataset of means, medians, etc. The Collapse - Make dataset of summary statistics dialog box will appear. 2. On the Main tab in the Statistics: box for 1: change “mean” to “sum” by clicking on the dropdown arrow. In the Variables: box select cprod_tt . 64
Stata 11 Sample Session Section 2 – Restructuring Data Files – Table Lookup & Aggregation 3. Click on the Options tab and in the Grouping variables box, select district vil hh in that order because those variables represent the identifycation of an individual household. The Grouping variable(s) is used to specify the variables to be used for combining cases in the collapsed file. Any cases from the original file that have identical values for all 3 of the grouping variables will be combined into a single case in the collapsed file. We want the collapsed file to have one case per household, so we use the variables that identify a household in our survey— district, vil, and hh. 4. Click on the copy button, switch to the do-file editor, paste the command, switch back to the dialog box and click on Ok. 5. Add a comment to explain what you have done. The Stata command is collapse (sum) cprod_tt, by(district vil hh) In the Variables Window you should see only 4 variables. Look at the resulting file (click on the Data Browser tool). You should see only one case per household. The collapse command created a new variable cprod_tt, which we calculated by summing cprod_tt, total calories produced, across all cases (all the different food crops) for each household. The only variables which are contained in a collapsed file are the grouping variables and any new collapsed computed variables created (e.g. cprod_tt). Remember to close the browser before you continue. Stata automatically added a variable label which is the function and variable used to create the resulting new variable. You can look at the variable definitions using the describe command. The computed variable cprod_tt does not have a very descriptive label any more so we need to change the label to reflect what the variable is. 1. Click on Data, then Data utilities, then Label utilities, then Label variable. 2. In the Variable: box cprod_tt should be selected 3. In the New variable label (may be up to 80 characters) box, type Calories Produced in Staple Foods 4. Click on the copy button, switch to the do-file editor, paste the command, switch back to the dialog box and click on the Ok button. 65
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