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
______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
