Can any one tell how to determine minimum sample size in a prospective cohort study when the cohort is not divided into exposure and non exposure groups? Thank you!!!!!
It really depends on what your hypothesis is. If you expect a change in outcome over time, then the sample size calculation can be based on repeated measures ANOVA.
Dear Behailu
The sample size depends on the type of your population, is it finite population or infinite population.
In the case of finite population, you can use Slovin formula:
n= N / ( 1+ N * e^2 )
But in the case of an infinite population:
The sample size for any study depends on the standard deviation of the variable ( from previous studies ) and the margin of error you decided. The formula:
n= ( Z^2 * S^2) / E^2
where : Z ( 1.96 for 0.05 and 2.58 for 0.01 )
S = standard deviation from previous studies or pilot study
E = significant level
Also, you can use software to calculate sample size ( SPSS , Minitab , G*power )
My advice to use G*Power . G*Power software is effective tool to calculate sample size for many ranges of experiments. Also, you can determine effect size and power of the test, G*Power is free to download and easy to use after reading the manual, the download link:
http://download.cnet.com/G-Power/3000-2054_4-10647044.html
The link to download G*Power manual:
http://www.gpower.hhu.de/fileadmin/redaktion/Fakultaeten/Mathematisch-Naturwissenschaftliche_Fakultaet/Psychologie/AAP/gpower/GPowerManual.pdf
I hope that be useful for you.
Good Luck
Dear Behailu:
I have the same problem you had. I can't find any formula or software to calculate the sample size for a retrospective single-arm cohort study (not divided into exposed and unexposed cohort) in which the population is finite. Would you be so kind as to tell me how you solved this problem? I would greatly appreciate it.
Dear Behailu and Sol Camus,
I am also experiencing the same problem. Please can you share your experience on how you handled this problem?
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