Re: [R] summing values according to a factor

From: Liaw, Andy <andy_liaw_at_merck.com>
Date: Mon, 07 May 2007 09:53:16 -0400

Howdy!

I guess what you want to do is compare Q1/T1 among the sections? If you want to compute the sum of Q1/T1 by Section, you can do something like:

sum.by.section <- with(mydata, tapply(Q1/T1, section, sum))

Substitute sum with anything you want to compute.

Cheers,
Andy

From: Salvatore Enrico Indiogine
>
> Greetings!
>
> I have exam scores of students in several sections. The data
> looks like:
>
> StuNum Section Q1 T1
> 111 502 45 123
> 112 502 23 123
> 113 503 58 123
> 114 504 63 123
> 115 504 83 123
> ......
>
> where Q1 is the score for question 1 and T1 is the maximum possible
> score for question 1
>
> I need to check whether the section has an effect on the scores. I
> thought about using chisq.test and calculate the sums of scores per
> section.
>
> I think that I have to use apply() but I am lost here.
>
> Thanks in advance,
> Enrico
>
> --
> Enrico Indiogine
>
> Mathematics Education
> Texas A&M University
> hindiogine_at_gmail.com
>
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>
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>



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