Determination of Sample Sizefor Estimating Means (5)• Estimating 2– The formulas for sample size require knowledge of 2but, as has been pointed out, the population variance is,as a rule, unknown. As a result, 2 has to be estimated.The most frequently used sources of estimates for 2 arethe following:• A pilot or preliminary sample may be drawn from the population, andthe variance computed from this sample may be used as an estimate of2. Observations used in the pilot sample may be counted as part ofthe final sample, so that n (the computed sample size) -n1 (the pilotsample size) = n2 (the number of observations needed to satisfy thetotal sample size requirement).• Estimates of 2 may be available from previous or similar studies.• If it is thought that the population from which the sample is to bedrawn is approximately normally distributed, one may use the factthat the range is approximately equal to six standard deviations andcompute R/6 This method requires some knowledge of the smallestand largest value of the variable in the population. 60