[R] chisq.test on samples of different lengths

[David Winsemius]

On Aug 24, 2010, at 10:12 AM, Marino Taussig De Bodonia, Agnese wrote:

> Hello,
>
> I am trying to see whether there has been a significant difference  
> in whether people experienced damages from wildlife in two different  
> years. I therefore have two columns:
>
> year 1:
> yes
> no
> no
> no
> yes
> yes
> no
>
> year 2:
> no
> yes
> no
> yes
>
> I wanted to do a chisq.test, but if I enter it this way:
>
> chisq.test(year1, year2)
>
> I get the error saying the columns are two different lengths. So  
> then I tried doing:
>
> damages<-matrix(c(3,4, 2,2), ncol=2, dimnames=list(answer=c("yes",  
> "no"), year=c("year1", year2)))
> chisq.test(damages)

Which should throw an error because year2 is not quoted. Consider  
using prop.test:

?proptest

So your matrix is the transpose of what is needed for prop.test, at  
least as I read the docs:

 > damages<-matrix(c(3,4, 2,2), ncol=2, byrow=TRUE,  
dimnames=list(year=c("year1", "year2"),success=c("yes", "no")))
 > damages
        success
year    yes no
   year1   3  4
   year2   2  2
 > prop.test(damages)

	2-sample test for equality of proportions with continuity correction

data:  damages
X-squared = 0, df = 1, p-value = 1
alternative hypothesis: two.sided
95 percent confidence interval:
  -0.7548099  0.6119528
sample estimates:
    prop 1    prop 2
0.4285714 0.5000000

Warning message:
In prop.test(damages) : Chi-squared approximation may be incorrect


>
> Does that make sense? Should I maybe be doing a different test  
> instead?
>
> Any help would be appreciated, thank you.
>
> Agnese
>
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David Winsemius, MD
West Hartford, CT