I am comparing my study findings to that of the general population. I already have a sample of 700 participants how best can I justify my study sample.
Alyona,
The sample size is usually determined according to the intended power of your statistical analysis/inference. Most statistical software (JMP, MINITAB, R, etc.) provide some kind of tool to determine the appropriate sample size based on the intended power of the test. I recommend looking into Montgomery's Design and Analysis of Experiments book for more specific information on determining the appropriate sample size.
Hi Armin,
Thank you for your reply.
I only approximated the sample size based on the number of participants that attended the clinic. Can I now do a Post -Hoc analysis using G-Power soft ware to determine whether our sample size is large enough to provide statistical significance to detect meaningful effect? Taking into account that my comparison group is the general population?
Alyona,
Yes, you can perform the post-hoc using software to determine that.
Dear Alyona
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 the attached table to estimate the suitable sample size. Or 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
- Badges
- Science topic
- Similar topics
- Mathematics
- Statistics
- Sampling Design
- Sample Size