Contract command

[Sudipto Banerjee]

I need to calculate the frequencies of a categorical variable across different time periods. I want to create a new data set of those frequencies. I was trying to use the "contract" command, but it does not have the by option. Can anybody suggest something. Can I use "collapse" for this? Any help would be appreciated.

[Roberto Ferrer]

I guess so:

clear
set more off

*----- example data -----

sysuse auto
keep rep78 foreign

// fails
capture bysort foreign : contract rep78
list

// works
gen rep78_2 = rep78
collapse (count) count=rep78, by(rep78_2 foreign)

// list
sort foreign rep78_2
list, sepby(foreign)

Watch out for the missings.

There are other ways too.

[Nick Cox]

If you want

 
bysort avar : contract bvar

then

 
contract avar bvar

is surely the same thing, as it produces frequencies according to cross-combinations of the variables; and so too more generally for varlists in each position. Otherwise put, as the original author of contract I must have missed something if you want something else.

[Sudipto Banerjee]

Thanks everyone. Nick's suggestion worked perfectly.