For a standard normal curve, find the z-score that separates the bottom 90% from the top 10%.

Solution:

The top 10% and the bottom 90% can be represented on the normal curve as shown below:

From the tables representing the normal distribution curve the 90% area under the curve corresponds to the z value of 1.28 as shown above. The area beyond the z = 1.28 is the 10% area. In the tables the closest value to 0.9 (90%) is 0.8997. For this value of area the z-score value is 1.28

For a standard normal curve, find the z-score that separates the bottom 90% from the top 10%.

Summary:

For a standard normal curve, the z-score that separates the bottom 90% from the top 10% is z = 1.28