In order to be considered as including all formula_7 observations in the sample, the probability band (centered on the mean) must only account for formula_8 samples (if formula_9 then only 2.5 of the samples must be accounted for in the probability band). In reality we cannot have partial samples so formula_8 (2.5 for formula_9) is approximately formula_7. Anything less than formula_8 is approximately formula_14 (2 if formula_9) and is not valid because we want to find the probability band that contains formula_7 observations, not formula_14 samples. In short, we are looking for the probability, formula_18, that is equal to formula_8 out of formula_7 samples (Eq.2).