Question

Let Z be the standard normal random variable. Use a probability calculator to answer the following questions: What is the probability Z will be within one standard deviation of average?

Answer 1

0.6826 = 68.26% probability Z will be within one standard deviation of average.

Explanation:

Normal Probability Distribution

Problems of normal distributions can be solved using the z-score formula.

In a set with mean
`\mu` and standard deviation
`\sigma`, the z-score of a measure X is given by:

`Z = (X - \mu)/(\sigma)`

The Z-score measures how many standard deviations the measure is from the mean. After finding the Z-score, we look at the z-score table and find the p-value associated with this z-score. This p-value is the probability that the value of the measure is smaller than X, that is, the percentile of X. Subtracting 1 by the p-value, we get the probability that the value of the measure is greater than X.

What is the probability Z will be within one standard deviation of average?

This is the p-value of Z = 1 subtracted by the p-value of Z = -1.

Z = 1 has a p-value of 0.8413.

Z = -1 has a p-value of 0.1587.

0.8413 - 0.1587 = 0.6826

0.6826 = 68.26% probability Z will be within one standard deviation of average.