Dear Oluwasola

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 estimate the suitable sample size by using 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

(T sqr PQ / D sqr) * DE

where T is 1.96, P the prevalence level of variable under study, (if unknown take 0.5)

Q= 1-P

D = margin of error (u may take 0.5)

DE= Design effect which may be 1 or 1.5 or 2 (often we take 1.5, whereas 2 being the best quality of sample size ie. close to SRS)