Suppose you have a normal random variable x with mu=50 and s=15.  Find the probability that x will fall within the interval 30<x<60.

1 Answer | Add Yours

embizze's profile pic

embizze | High School Teacher | (Level 1) Educator Emeritus

Posted on

Suppose you have a normal random variable x with mu=50 and s=15.  Find the probability that x will fall within the interval 30<x<60.

The mean is given as 50 and the standard deviation as 15. We convert the raw data to z scores using `z=(a-mu)/sigma` where a is the data point, `mu` is the mean, and `sigma` the standard deviation.

Thus 30 converts to z score of -4/3, and 60 converts to 2/3.

We want to know the area under the standard normal curve between these two z values, as the area is the probability we are seeking.

Using a TI-83 I got the area to be .6563. Using the z-table found in most statistics books and finding the values closest to -4/3 and 2/3 I got the area to the left of -4/3 to be .0918, and the area to the left of 2/3 to be .7486. Taking .7486-.0918 gives .6568.

So the probability that a given x will fall in the range is approximately .6563 (The calculator computes more than the standard 4 digits in a table, so I use this value.)

We’ve answered 318,982 questions. We can answer yours, too.

Ask a question