[R] Aggregating dataset to means/day

From: OA Fatnes <ofa082_at_gmail.com>
Date: Wed, 16 Mar 2011 11:17:11 +0100


Hi,

I have a dataset with many observations some days while only one others. I would like to calculate a mean value per day and then do regression analysis on the means.

This is what I have:

Year Day Time herring.density

2007    47     10.36           2.2
2007    47     11.50           1.1
2007    47     14.24           1.4
2007    66      9.35            2.5

This is what I want:

Year Day herring.density

2007     47         1.57
2007     66         2.25

I would also might like to extract means between time 10-16 (h) so that

Year Day herring.density
2007 47 1.25

Any idea on how to do this?

I have tried

means07<-tapply(herring.density,Day,mean)

Then I get means for every day, but means07 won't fit when plotted against days, because it is now shorter than Day. I also get a lot of NA values where there are only 1 observation that day...

Help would be much appreciated!

Cheers,

Ole Andreas

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