It is not always possible to estimate the sample sizes needed in health services research because special formulas are needed, and the necessary data may not be available to use in the formulas. We provide some useful formulas for the sample size required in comparing the means of two groups. These include the special case where the two groups are not of equal size either because one is known to have a higher variability or because one group has already been chosen and its size is thus fixed. We also explore the relationship of the mean to the standard deviation for utilization measures, so that the latter can be estimated from the former for use in the equations. In general, the coefficient of variation is on the order of 2, suggesting that the standard deviation may be crudely estimated as twice the mean. The optimal follow-up period is also calculated.



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