[R] Sample size calculation proportions with EpiR: Discrepancy to other calculators
tlumley at u.washington.edu
Tue May 26 16:43:09 CEST 2009
On Tue, 26 May 2009, Chuck Cleland wrote:
> On 5/26/2009 2:53 AM, Karl Knoblick wrote:
>> 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
>> ration group 1 and 2: 1.0
>> I have done this in the following way:
>> 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")
>>  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 Lumley Assoc. Professor, Biostatistics
tlumley at u.washington.edu University of Washington, Seattle
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