st: calculating mean without own observation

[Guo Xu <digitalepourpre@gmail.com>]

Dear Stata list,

How do I calculate a mean (or any other summary statistic) excluding
the *current* observation?

For example, I have following data:

i      value      category
1     5            1
2     5            1
3     10          1
4      2           2
5      2           2

I would like to calculate the mean for each category (egen value_mean
= mean(value), by(category)), but exclude the i-th observation:
For i=1, for example, the mean value by category 1 would be (5+10)/2.
For i=3, it would be (5+5)/2. For i=4, it would be 2/1.

I guess there must be some simple *if* condition for this
manipulation, but I failed to find it - would be most grateful for
help.

Thanks,

Guo
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