[R] summing values according to a factor

[Liaw, Andy]

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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