[R] binom.test and small N
Tony Long
tdlong at uci.edu
Fri Mar 22 18:58:41 CET 2002
running R 1.4.1 on MAC and 1.2.2 on Linux
When I use run binom.test with small N the results are a little
perplexing to me
>binom.test(9,20,p=0.5)
gives the below plus other stuff
95 percent confidence interval:
0.2305779 0.6847219
Now:
>pbiom(9,20,0.6847219)
[1] 0.02499998 # i.e., lower 2.5% of distribution
>pbinom(9,20,0.2305779)
[1] 0.9923132
>pbinom(8,20,0.2305779)
[1] 0.975 # i.e., upper 2.5% of distribution
I would think I generally want a 95% CI on the observed 9 out of 20,
not 8 out of 20 LB and 9 out of 20 UB. I am missing something or is
this strictly a function of the beta approximation used in the source
code??
--
Tony Long
Ecology and Evolutionary Biology
Steinhaus Hall
University of California at Irvine
Irvine, CA
92697-2525
Tel: (949) 824-2562 (office)
Tel: (949) 824-5994 (lab)
Fax: (949) 824-2181
email: tdlong at uci.edu
http://hjmuller.bio.uci.edu/~labhome/
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