Creating sub-group cohorts to analyse in a large cross-sectional population

[George Freer]

Hello,
For my project I have a large dataset including including individuals over many years. To be eligible for inclusion in the dataset all included participants have a medical condition, call it x. I need to create a number of yearly sub-group populations that includes all the participants with x up to the first day of the year. E.g, for the sub-group year 2002, all the participants diagnosed (index date) with x before 01/01/2002 whilst excluding those with a diagnosis after this date or death/transfer out prior to 31/12/2002.
I'm trying to figure out how to best create these sub-groups in stata so I can subsequently analyse their characteristics.
Any advice is much appreciated.
Best regards

[Ken Chui]

Not a lot details to work on, but "inrange" and "if" could be the first step:

clear
input date conx
21910 1
21915 1
22280 1
22280 1
21910 0
21915 0
22280 0
22280 0
end
format date %td

gen include = conx == 1 & inrange(date, date("01/01/2020", "MDY"), date("12/31/2020", "MDY"))

list, sep(0)