How to calculate the acceptable number of subjects (sample size) in a survey?
hi,
in the following site, you can calculate the sample size by determining the confidence interval and error level.
http://www.surveysystem.com/sscalc.htm
hi,
in the following site, you can calculate the sample size by determining the confidence interval and error level.
http://www.surveysystem.com/sscalc.htm
Abdullah -
Calculators you find on the internet generally assume you are only looking at yes/no data. Often they assume worst case p=q=0.5, though I see you can change that for the calculator shown above. That is in place of estimating the standard deviation, which is important. If your sample size is a large part of the population, then the finite population correction (fpc) factor is generally desired. If you have continuous data, then you need to consider other 'formulae.' The type of data and the sample design are important.
You are using randomization I assume.
Standard deviation is generally needed before looking at standard error of the means.
There are chapters on sample size in the following two books, and other necessary information:
Cochran, W.G(1977), Sampling Techniques, 3rd ed., John Wiley & Sons.
Blair, E. and Blair, J(2015), Applied Survey Sampling, Sage Publications.
There are many other good survey sampling books that may help you. If you are working with other kinds of quantitative or qualitative data, there may be other considerations.
You may want to start with a pilot study.
Cheers - Jim
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