[R] significance in difference of proportions

Arne.Muller@aventis.com Arne.Muller at aventis.com
Thu Nov 27 17:04:35 CET 2003


I'm looking for some guidance with the following problem:

I've 2 samples A (111 items) and B (10 items) drawn from the same unknown
population. Witihn A I find 9 "positives" and in B 0 positives. I'd like to
know if the 2 samples A and B are different, ie is there a way to find out
whether the number of "positives" is significantly different in A and B?

I'm currently using prop.test, but unfortunately some of my data contains
less than 5 items in a group (like in the example above), and the test
statistics may not hold:

> prop.test(c(9,0), c(111,10))

        2-sample test for equality of proportions with continuity correction

data:  c(9, 0) out of c(111, 10) 
X-squared = 0.0941, df = 1, p-value = 0.759
alternative hypothesis: two.sided 
95 percent confidence interval:
 -0.02420252  0.18636468 
sample estimates:
    prop 1     prop 2 
0.08108108 0.00000000 

Warning message: 
Chi-squared approximation may be incorrect in: prop.test(c(9, 0), c(111, 10))

Do you have suggestions for an alternative test?
	many thanks for your help,
	+kind regards,


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