Hello, everyone,

I have a matrix like following:

school  value
A          .1
A          .2
A          .15
A          .2
B          .3
B          .5
C          .3
C          .3
C          .4
C          .5
C          .6
C          .9
C          1

I want to get the mean 'value' for each 'school', but each school has
different length. Is there any way to do this fast? Because my data has
hundreds of thousands of rows.

Thank you,

Cindy

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