Panel data: keep rows if condition is met

[Sara Khan]

Hello,

I have posted my data below. Some individuals have 2 records. I would like to keep those records where an individual has a single record only or where Index==2 (i.e., the 2nd record). I manually created the variable keep to show the records I wish to retain. I would be grateful for advice on how to write code for the variable keep. Thank you!

* Example generated by -dataex-. To install: ssc install dataex
clear
input float(ID Index keep)
1 1 1
2 1 .
2 2 1
3 1 .
3 2 1
4 1 1
5 1 1
end

[Andrew Musau]

bys ID: keep if _N==1| Index==2

The "or" operator is the pipe. _N defines the number of observations per ID group. See

help operators

[Sara Khan]

Thank you for replying so quickly! Perfect for my needs.

[Nick Cox]

bysort ID (Index) : keep if _n == _N

would work here too.