[R] Calculate monthly means

[KATSCHKE, ADRIAN CIV DFAS]

You could use the by() function after a little data manipulation.

The first line will create a field just of the date portion of your datetime field. Then you can use the by() function to use the indices you desire to calculate the mean.

mSamp$cDT <- chron(unlist(strsplit(as.character(mSamp$Collection_Date), split=" "))[seq(1,nrow(mSamp),2)], format="y-m-d", out.format="m/d/y")


with(mSamp, by(Value, list(years(cDT), months(cDT)), mean, na.rm=TRUE))

I like using the chron package for my dates and times.

Adrian 

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-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Carl Nim
Sent: Tuesday, March 15, 2011 10:25 AM
To: r-help at r-project.org
Subject: [R] Calculate monthly means

I am trying to calculate monthly means by year of phosphates and nitrates from a multi year data set. Can anybody help me out with the most effective way to do this?

My data looks like this:

Collection_Date                   Test.Name              Value
2000-01-24 17:00:00            Phosphate               0.108
2000-01-24 17:00:00            Nitrate                     0.037
2001-11-12 08:45:00            Phosphate               0.45
...


Thanks and sorry for the blatantly "newbie" question.




      
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