[R] by function ??

[L.A.]

I'm just learning and this is probably very simple, but I'm stuck.
   I'm trying to understand the by().
This works.
by(TestData, LEAID, summary)

But, This doesn't.

by(TestData, LEAID, median(RATIO))


ERROR: could not find function "FUN"

HELP!
Thanks,
LA
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