Example R programs and commands
21. Fisher exact test for independence in 2x2 contingency tables
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# All other left-justified lines are my input.
# All other indented lines are the R program output.
# FISHER EXACT TEST in R
#
# Example data:
# Area With rabies Without rabies
# ---- ----------- --------------
# E 16 11
# W 6 17
# Read the 2x2 contingency table, r=2 rows (X values), c=2 columns (Y values):
data<-scan()
16 11
6 17
tab <- t(matrix(data, nrow=2,ncol=2)) # transpose, as usual
tab # view the table to make sure it is right
[,1] [,2]
[1,] 16 11
[2,] 6 17
fisher.test(tab) # performs a test of independence for matrix input.
Fisher's Exact Test for Count Data
data: tab
p-value = 0.02437
# ==> REJECT the null hypothesis that X,Y (area,rabies) are independent at
# the 0.05 level, namely that the odds of finding rabies is the same in
# area E and in area W.
alternative hypothesis: true odds ratio is not equal to 1
95 percent confidence interval:
1.072816 16.715195
sample estimates:
odds ratio
3.998220
# Conclude that the odds of rabies in E is about 4 times the odds of rabies
# in area W.