Question

Heeeeeelp

Find the z score that corresponds to P99, the 99th percentile of a standard normal distribution curve.

Answer 1

`Z = 2.325`.

Explanation:

Problems of normally distributed samples can be solved using the z-score formula.

In a set with mean
`\mu` and standard deviation
`\sigma`, the zscore 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 pvalue, we get the probability that the value of the measure is greater than X.

Find the z score that corresponds to P99, the 99th percentile of a standard normal distribution curve.

This is the value of Z when X has a pvalue of 0.99. This is between 2.32 and 2.33, so the answer is
`Z = 2.325`.

Answer 2

about 2.33

Explanation:

The value can be found from a probability table, any of several web sites, your graphing calculator, most spreadsheet programs, or any of several phone or tablet apps.

A web site result is shown below. (I have had trouble in the past reconciling its results with other sources.) One of my phone apps gives the z-value as about ...

2.26347874

which is in agreement with my graphing calculator.