如果语句Stata

Question

I'm trying to build the following if statement in Stata: I want Stata to restrict my sample with the following conditions keep if distance > 50 & distance < 60 but only if the binary variable category = 0 . How can I achieve this?

Answer 1

You can add an extra condition in your if-statement.

keep if category == 1 | (category == 0 & distance > 50 & distance < 60)

Answer 2

If you are saying you want to restrict the sample to all observations that are category 0 and are also between 50 and 60 in distance, that is:

keep if (category == 0 & distance > 50 & distance < 60)

Answer 3

If you would like to maintain both types of observations in your dataset I recommend you create a new dummy variable called restricted_sample . After that, you could calculate statistics with both categories or each of them separately. You can create this variable according to your requirements in the following way:

gen restricted_sample = 1 if category == 0 & distance > 50 & distance < 60 & !missing(distance) & !missing(category) 
replace restricted_sample = 0 if restricted_sample == .& !missing(distance) & !missing(category) 

Then you can obtain statistics considering the different subgroups:

* All 
sum distance

* For each group
sum distance if restricted_sample == 1
sum distance if restricted_sample == 0

You can test the code with the following dataset. This dataset includes some missing values also.

clear all 
set obs 100
gen distance = runiform(1,100)
gen category = 1 if _n > 50
replace category = 0 if _n < 50 
replace distance = . if mod(_n, 7) == 0
replace category = . if mod(_n, 13) == 0 

I hope it will be useful for you.