The accompanying sample consisting of n = 20 observations on dielectric breakdown voltage of a piece of epoxy resin appeared in the article "Maximum Likelihood Estimation in the 3Parameter Weibull Distribution (IEEE Trans. on Dielectrics and Elec. Insul., 1996: 43-55). The values of (i -.5) / n for which z percentiles are needed are (1 -.5) / 20 = .025, (2 - .5) / 20 = .075,..., and .975. Would you feel comfortable estimating population mean voltage using a method that assumed a normal population distribution?
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