For more details see sample size determination.

To determine an appropriate sample size you must first declare an

Example: To obtain a margin of error of 5 for a variable with a standard deviation of 15, n = (4)(15^2)/(5^2) = 36.

This method is applied to estimating a mean difference based on paired samples (µ_d) by using the standard deviation of the

You should put considerable effort into getting a good estimate of the standard deviation of the variable you are studying since sample size calculations depend on this fact. Such estimates come from prior studies, pilot studies, and “Gestalt” (a combination of sources that contribute to knowledge about the variable).

**Sample Size Requirements for Estimating a Mean or Mean Difference**To determine an appropriate sample size you must first declare an

**acceptable margin of error d**. Recall that margin of error d is the wiggle room around the point estimate. This is equal to**half the confidence interval width**. When estimating µ with 95% confidence useExample: To obtain a margin of error of 5 for a variable with a standard deviation of 15, n = (4)(15^2)/(5^2) = 36.

This method is applied to estimating a mean difference based on paired samples (µ_d) by using the standard deviation of the

**DELTA variable**(s_d) in your formula:You should put considerable effort into getting a good estimate of the standard deviation of the variable you are studying since sample size calculations depend on this fact. Such estimates come from prior studies, pilot studies, and “Gestalt” (a combination of sources that contribute to knowledge about the variable).