Without a sample standard deviation for an estimator of the population standard deviation u in a sample size computations, we often use some functions of a sample range (R) or interquartile range (IQR) by an estimator of $sigma$. In order to avoid under-powered studies, these estimates must have a high probability of being greater than or equal to $sigma$. In this paper, these probabilities of being greater than or equal to $sigma$ are estimated for IQR for various parents distributions, and are compared with the probabilities for R/4 (Browne 2001). Alternative divisors (K) are explored and discussed for which the probabilities of R/K and IQR/K being greater than or equal to $sigma$ is at least 95%.