Calculating mean by selecting from two columns in a data frame

Question

I would like to calculate the mean of a data.frame by two variables. See an example of the data.frame (extraction) below:

         Station Time Year Month Value         
         ARO 199501 1995     1      69
         ARO 199502 1995     2      87
         ARO 199503 1995     3     107
         ARO 199601 1996     1      35
         ARO 199602 1996     2      46
         ARO 199603 1996     3      50
         ANT 200401 2004     1      87
         ANT 200402 2004     2     115
         ANT 200403 2004     3     110
         ANT 200501 2005     1      80
         ANT 200502 2005     2     122
         ANT 200503 2005     3     107

To be more detailed: I would like to calculate the mean value for each Station and Month, so e.g. Mean for ARO in Month 1 = (69+35)/2, Mean for ANT in Month 1 = (87+80)/2

The year doesn't matter since I would like to have the mean for a period of 20 years for every month and station.

My dataframe is huge with 61 stations and 12 months for a timeseries of 20 years each.

I tried several things like split or aggregate and ddply but none of it worked. At the end I would like to have a new data frame like to following:

Station Month Valuemean
ARO 1 52
ARO 2 66.5
ARO 3 78.5
ANT 1 83.5
ANT 2 118.5
ANT 3 108.5

Would be great if you have some ideas to realize it. Thanks a lot!

PS: I'm a R beginner ;)

Answer 1

You can use data.table package:

library(data.table)
setDT(df)[,mean(Value), by=list(Month, Station)]

Answer 2

assuming you data is named df, you can try aggregate

 aggregate(Value~Month+Station, data=df, FUN = mean)
  Month Station Value
1     1     ANT  83.5
2     2     ANT 118.5
3     3     ANT 108.5
4     1     ARO  52.0
5     2     ARO  66.5
6     3     ARO  78.5

Answer 3

Using the dplyr package, if your data.frame is called dat:

library(dplyr)
means <- dat %>%
  group_by(Station, Month) %>%
  summarise(Valuemean = mean(Value, na.rm = TRUE))