The study is a prospective cohort study to find out the association between early body fat percentage and the occurence of gestational diabetes mellitus.
For sample size normally I would need
•α = 5%
•Power = 0.80
•Ratio (unexposed: exposed) = ?
•% outcome in unexposed group = ?
•% outcome in exposed group = ?
•Risk Ratio of ?
The problem:So far studies have only evaluated the association between BMI (pre -gestational or early in pregnancy) and GDM, so I do not have a group of exposed Vs non exposed.
The plan is to divide the results of body fat percentages in three categories of percentage of body fat and use the leanest group as a "reference group".Thee is one study that used the same inclusion criteria and similar strategies but with other objectives, they recruited 250 women so we expect torecruit the same amount. How could calculate the power ahieved with this?
J Clin Epidemiol. 2015 Nov;68(11):1375-9. doi: 10.1016/j.jclinepi.2015.04.014. Epub 2015 Jun 6.
A simple formula for the calculation of sample size in pilot studies.
Viechtbauer W1, Smits L2, Kotz D3, Budé L4, Spigt M5, Serroyen J6, Crutzen R7.
Abstract
One of the goals of a pilot study is to identify unforeseen problems, such as ambiguous inclusion or exclusion criteria or misinterpretations of questionnaire items. Although sample size calculation methods for pilot studies have been proposed, none of them are directed at the goal of problem detection. In this article, we present a simple formula to calculate the sample size needed to be able to identify, with a chosen level of confidence, problems that may arise with a given probability. If a problem exists with 5% probability in a potential study participant, the problem will almost certainly be identified (with 95% confidence) in a pilot study including 59 participants.
KEYWORDS:
Pilot study; Problem detection; Rule of three; Sample size; Unforeseen problems
PMID: 26146089 DOI: 10.1016/j.jclinepi.2015.04.014 [Indexed for MEDLINE]
The criteria for selecting the sample size when working with human beings is very varied. These in front of gestational diabetes mellitus and early body fat percentage and 250 patients seem a quite large number. Annex some articles that could help you.
I think this link will be helpful for you
http://sphweb.bumc.bu.edu/otlt/mph-modules/bs/bs704_power/BS704_Power_print.html
Thank you very much for your interest and support with this doubt!! I will check the links and papers and try to make the calculation.
ind regards,
Laura
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