..

Recall from Chapter 2 that we presented the empirical rule, which states as a rough guide that u to covers about 68% of the range, while he # 20 covers about 95% of the range. For all practical purposes, almost none of the range is outside u + 30. This useful approximation tool for interpretations based on descriptive statistics is based on the nor- mal distribution Probabilities can also be computed by using Equation 5.14. Finding Probabilities for Normally Distributed Random Variables Let X be a normally distributed random variable with mean p and variance o? Then random variable Z = (X)/o has a standard normal distribution of Z-N(0,1). It follows that, ifa and bare any possible values of X with a