On Tue, 26 May 2009, Chuck Cleland wrote:
On 5/26/2009 2:53 AM, Karl Knoblick wrote:
Hallo!
I have done a sample size calculation for proportions with EpiR. The input is:
treatment group rate p=0.65
control group rate p=0.50
significance level 0.95
power 0.80
two-sided
ration group 1 and 2: 1.0
I have done this in the following way:
library(epiR)
epi.studysize(treat = 0.65, control = 0.5, n = NA, sigma = NA, power = 0.80,
r = 1, conf.level = 0.95, sided.test = 2, method = "proportions")
Result:
$n
[1] 82
PASS 2002 and NQuery give both 170 subjects per group without continuity
correction. With continuity correction 183 per group.
Looking at http://statpages.org/proppowr.html I get 182 subjects per group
(with continuity correction, I admit).
What am I doing wrong? Can anybody explain this?
epi.studysize(treat = .65, control = .50, n = NA, sigma = NA,
power = 0.80, r = 1, conf.level = 0.95, sided.test = 2, method = "cohort")
gives the same sample size as PASS 2002 and NQuery (170 per group).
And simulation confirms that the larger numbers are correct. I don't know
what is happening with epi.studysize(,method="proportion").
epi.studysize(,method="cohort") doesn't seem exactly appropriate, since
judging from the example on the help page the inputs are supposed to be
cumulative incidence rather than probabilities.
-thomas
Thomas Lumley Assoc. Professor, Biostatistics
tlum...@u.washington.edu University of Washington, Seattle
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