Conditional Statistics

...if I had the closing prices of the
S&P from 01/01/1990-12/31/1990, how could I get the average price of
the S&P from 02/01/1990-03/15/1990? Or the average price of the S&P on
Mondays (assuming a dummy var is created for 1 = Monday, 0 = else). 

tapply has already been referred to. You may also find aggregate() useful, as it gives you back a data frame that includes the conditioning variables if you tell it to. Alse ave, if you want to do something like mean-centring a data set based on group means rather than the grad mean.

S Ellison

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