[R] Within Subject ANOVA question

[tsunhin wong]

I have reformatted the data input in the following ways, and I get the
normal results:
>subject<-factor(c("s1","s2","s3","s4","s5","s6","s7","s8","s9","s10","s1","s2","s3","s4","s5","s6","s7","s8","s9","s10","s1","s2","s3","s4","s5","s6","s7","s8","s9","s10"))
>condition<-factor(c("c1","c1","c1","c1","c1","c1","c1","c1","c1","c1","c2","c2","c2","c2","c2","c2","c2","c2","c2","c2","c5","c5","c5","c5","c5","c5","c5","c5","c5","c5"))
> recall<-c(10,6,11,22,16,15,1,12,9,8,13,8,14,23,18,17,1,15,12,9,13,8,14,25,20,17,4,17,12,12)
> example<-data.frame(subject,condition,recall)
> aov.recall <- aov(recall~condition + Error(subject/condition),data=example)
> summary(aov.recall)
Error: subject
          Df Sum Sq Mean Sq F value Pr(>F)
Residuals  9 942.53  104.73

Error: subject:condition
          Df Sum Sq Mean Sq F value    Pr(>F)
condition  2 52.267  26.133  42.506 1.519e-07 ***
Residuals 18 11.067   0.615
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

But I have some follow-up questions:
1. In aov, why IVs (subject & condition) cannot be in number format?
2. Why I need to use factor() on IVs (subject & condition) but I
cannot use factor() on DV (recall)?

Thanks a lot!

- John


On Mon, Jun 1, 2009 at 6:31 PM, tsunhin wong <thjwong at gmail.com> wrote:
> Dear R users,
>
> I have copied for following table from an article on "Using confidence
> intervals in within-subject designs":
>
> Subject 1sec 2sec 5sec
> 1 10 13 13 12.00
> 2 6 8 8 7.33
> 3 11 14 14 13.00
> 4 22 23 25 23.33
> 5 16 18 20 18.00
> 6 15 17 17 16.33
> 7 1 1 4 2.00
> 8 12 15 17 14.67
> 9 9 12 12 11.00
> 10 8 9 12 9.67
>
> I rearranged the data this way:
>>subject<-factor(c(1,2,3,4,5,6,7,8,9,10,1,2,3,4,5,6,7,8,9,10,1,2,3,4,5,6,7,8,9,10))
>>condition<-factor(c(1,1,1,1,1,1,1,1,1,1,2,2,2,2,2,2,2,2,2,2,5,5,5,5,5,5,5,5,5,5))
>>recall<-factor(c(10,6,11,22,16,15,1,12,9,8,13,8,14,23,18,17,1,15,12,9,13,8,14,25,20,17,4,17,12,12))
>>example<-cbind(subject,condition,recall)
>
> Using ANOVA (aov), I should have DF for condition = 2, DF of subjects
> = 9, Interaction DF = 18
> And a term for mean square of interaction. (0.61)
>
> But, I have something like below instead:
>>aov.recall <- aov(recall~condition + Error(subject/condition),data=as.data.frame(example))
>>summary(aov.recall)
>
> Error: subject
>          Df Sum Sq Mean Sq F value Pr(>F)
> Residuals  1 12.898  12.898
>
> Error: subject:condition
>          Df Sum Sq Mean Sq
> condition  1 34.505  34.505
>
> Error: Within
>          Df Sum Sq Mean Sq F value Pr(>F)
> condition  1   3.22    3.22  0.1378 0.7135
> Residuals 26 607.54   23.37
>
> The within-subject (repeated measure) anova of R seems to be a bit
> subtle for me, please point out the errors that I have made if you
> can.
> Thank you very much!
>
> - John
>