Hi All,

I'm really new to R and have figured out some basics. But I'm struggling to find a way to select only portions of my data to calculate summary statistics on. My data has approximately 250000 specimens which are divided into 6 size classes. I would like to be able to calculate for example the mean weight for each size class within each location separately. I thought I was on track with this code:

library(plyr)

ddply(Sandy_Point_collapsed, .(spsize), summarise, spweight=mean(spweight))

but the output was:

spsize spweight

1 1 0.4336955

2 2 0.4336955

3 3 0.4336955

4 4 0.4336955

5 5 0.4336955

6 6 0.4336955

so it looks like is is giving me a value for each size class but it's just repeating the mean for the entire dataset each time.

any suggestions would be greatly appreciated!