[R] chisq.test, basic question

[Ivan Frohne]

----- Original Message ----- 
From: "juli g. pausas" <juli at ceam.es>
To: "r-help" <r-help at stat.math.ethz.ch>
Sent: Tuesday, July 30, 2002 08:11
Subject: [R] chisq.test, basic question


> I've got the counts of a 2 x 2 contingency table, and I'd like to test
> the association:
> 
> m <-  matrix(c(15,28,32,135), 2, 2)
> colnames(m) <- c("R-", "R+"); rownames(m) <- c("P-", "P+")
> m
> #    R-  R+
> # P- 15  32
> # P+ 28 135
> 
> chisq.test(m)  # X-squared = 4.0027, df = 1, p-value = 0.04543
> 
> Is this the correct way to test association between P and R? (I haven't
> got the original data).

This the right way to do the 2X2 table chi-square test.

> My problem is that if I use percentage, then I get different results:
> 
> m2 <- 100*m/sum(m) #
> chisq.test(round(m2)) # X-squared = 1.5318, df = 1, p-value = 0.2158

This is incorrect.  Don't use percentages.  You could have the
same percentages with a total sample of 10, say, as with a
sample of 1000.

--Ivan Frohne


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