Hi all,
I started using R about 3 weeks ago, and now I've pretty much figured out how
to do the types of statistical modeling, graphs, tables etc. that I frequently
use (with zero background in computer languages or other statistical packages
that are similar to R like S or SAS!). So it's been a quite rewarding process
so far, and I thank you all R gurus for all your generous help!
That being said, my question is about applying a model or an analysis to
different groups based on a grouping variable. Below is the first six rows of
my data:
ID Group1 Group2 Mem Gen Chance MSELGM MSELVR MSELFM MSELRL MSELEL ADOS Age
1 1 1 1 75 50.0 50 53 52
62 57 56 3 25
2 2 1 1 75 12.5 50 46 48
47 52 55 2 30
3 3 1 1 25 37.5 50 48 43
52 63 63 3 24
4 4 1 1 25 37.5 50 51 62
52 59 54 0 31
5 5 1 1 50 87.5 50 45 58
42 46 43 6 31
6 6 1 1 100 100.0 50 45 80
49 69 63 1 31
Group1: First grouping variable
Group2: Second grouping variable
Mem: Memory trial
Gen: Generalization trial
MSEL: Mullen Scales of Early Learning (a scale measuring various skills in
little children). GM: Gross Motor Scale, VR: Visual Reception, FM: Fine Motor,
RL: receptive Language, EL: Expressive Language.
ADOS: An autism-specific measure.
First I wanted to do correlations between Generalization (variable Gen) and
expressive language (MSELEL) for each group of Group1. For this, I used lapply
or by functions which work just fine. Here is the code with
lapply: lapply(split(mydata, mydata$Group1), function(x){cor.test(x[,5],
x[,11], method = "pearson")})
Then I did regression. My DV is the variable Gen, and the IV is MSELEL. And
again I wanted to do this for each group. Here is the code I came up with for
each group:
fit1<-lm(Gen~ MSELEL, data=mydata, subset=mydata$Group1==1)
fit2<-lm(Gen~MSELEL, data=mydata, subset=mydata$Group1==2)
This works fine for regression, but when I used the "subset" function with the
correlation (e.g. cor.test (mydata$Gen, mydata$MSELEL, method="pearson",
subset=mydata$Group1==1) , it did not work. It just did the correlation for the
entire group and then used this for both groups. I was just curious as to why
subset function works with regression, but not with correlation. Any thoughts?
Thanks,
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