Sample Size Determination. Introduction. Integral part of vast majority of quantitative studies Important in ensuring validity, accuracy, reliability & scientific & ethical integrity Don’t give in to temptations of taking a shortcut
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Freiman JA, NEJM, 1978;299:690-4
nnew=n/(1-L)when L is the loss to f/u rate
Adjustment for unequal group size
Scenario: A randomized controlled trial has been planned to evaluate a brief psychological intervention in comparison to usual treatment in the reduction of suicidal ideation amongst patients presenting at hospital with deliberate self-poisoning. Suicidal ideation will be measured on the Beck scale; the standard deviation of this scale in a previous study was 7.7, and a difference of 5 points is considered to be of clinical importance. It is anticipated that around one third of patients may drop out of treatment
The formula for the sample size for comparison of 2 means (2-sided) is as follows
Suggested description of this sample size calculation
"A sample size of 38 in each group will be sufficient to detect a difference of 5 points on the Beck scale of suicidal ideation, assuming a standard deviation of 7.7 points, a power of 80%, and a significance level of 5%. This number has been increased to 60 per group (total of 120), to allow for a predicted drop-out from treatment of around one third"