[R] Subtraction with aggregate

[Jeff Newmiller]

What represents the difference when multiple values are present? sd?
-- 
Sent from my phone. Please excuse my brevity.

On July 28, 2016 1:40:16 PM PDT, Gang Chen <gangchen6 at gmail.com> wrote:
>With the following data in data.frame:
>
>subject   QM    emotion     yi
>  s1   75.1017   neutral  -75.928276
>  s2  -47.3512   neutral -178.295990
>  s3  -68.9016   neutral -134.753906
>  s1   17.2099  negative -104.168312
>  s2  -53.1114  negative -182.373474
>  s3  -33.0322  negative -137.420410
>
>I can obtain the average between the two emotions with
>
>mydata <- read.table('clipboard', header=TRUE)
>aggregate(mydata[,c('yi', 'QM')], by=list(subject=mydata$subject),
>mean)
>
>My question is, what is a nice way to get the difference between the
>two emotions?
>
>Thanks,
>Gang
>
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