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Sample Size for Surveys - True size II 
As it is better to err by excess, always chose the largest value between the estimators of the population variance, and use a beta error smaller than really needed. 
Thus if you err at least the estimated will have the precision required or more. 
Cost will be higher but better to spend a little more in observing more subjects, than have a set of estimates that are valueless because of their large confidence intervals.