st: Aggregate different parts of data selectively

[Chris Yang <bhpchg@gmail.com>]

Dear All,

I have a dataset of the following structure:

group no var1 var2 var3
1       1   3     2      1
1       2   2     0      0
1       3   3     0      1
1       4   0     1      1
1       5   1     1      2
...
2       1   3     2      1
2       2   3     0      0
2       3   1     1      1
2       4   0     2      1
2       5   0     0      2
...
3       1   1     1      1
3       2   2     3      1
3       3   2     1      1
...

I want to aggregate a subset of observations by group, e.g. aggregate
(using the mean) all the observations with `no` > 2 in group 1,
aggregate observations with `no` > 5 in group 2, aggregate
observations with `no` > 3 in group 3, and so on. And carry out
analysis using the aggregated data of different groups only.

I know that --collapse-- can do the averaging job within each group,
however, it will also replace all existing data with the aggregated
data. In my case, it would mean that I lose the data for all the other
groups once I collapse/aggregate for any single group.

A tedious way is to collapse and save respective groups individually,
and then merge the aggregated per-group data in the end. I wonder if
there is a more efficient way to achieve my goal here?

Your thoughts and suggestions are appreciated. Thanks.

Best regards,

Chris
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