Calculate the confidence interval of your research. Confidence interval is a plus and minus figure that represents how accurately the answers given by your sample size correlate to the answers given by the entire population. If your confidence level is 5, when 49 percent of your sample responds a particular way, then 44 (- 5) to 54 (+ 5) percent of the entire population would respond in the same way as the sample. Put this in decimal form: a confidence level of 5 will read .05.
Calculate your confidence level. Confidence level is how sure you are that your confidence interval is accurate. Standard confidence level is 95 percent. This means that you are 95 percent sure that your confidence interval is accurate.
Convert the confidence level to a Z-score, using the Z-score conversion table. A 95 percent confidence level equates to a 1.96 Z-score.
Estimate the percentage of your sample that will respond a given way. Convert this number to a decimal; this number is your proportion. If you estimate that 52 percent of the sample will respond a give way, then your proportion will be 0.52.
Compute the sample size needed by plugging the computed numbers into the following formula:
SSample Size = [Z(squared) * P * (1-P)] / C(squared)
Where Z is your computed Z-score (based on your confidence level), P is your estimated percentage of the sample who will respond a given way, and C is your computed confidence interval in decimal form.