st: Exclude non-positive values when using collapse

[Chris Yang <bhpchg@gmail.com>]

Dear All,

I want to collapse a dataset using the means of the variables.
However, in my data, some variables may contain value -1, and I want
to exclude only such values from the mean calculation for the variable
when collapsing.

For example, given the following dataset:

var1 var2 var3 group
0     1      1     1
0     -1     1     1
1     0      -1    1
1     1      1     1
1     1      1     2
1     0      0     2

After the collapse (by group), for group 1: I want var2 to be (1 + 0 +
1) / 3 and var3 to be (1 + 1 + 1) / 3, i.e. ignoring -1 values when
averaging.

How should I do that? Thanks a lot!

Best regards,

Chris
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