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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