1. Normal Probability Plot
Jim Bohan
Manheim Township School District
Lancaster PA

2. The Concept and the Purpose
The Normal Probability Plot is designed to give a visual indication of the normality of a data set; that is, whether a data set is normally distributed.
The plot can be used to suggest the normality of any univariate set of data. For example, to inspect whether the residuals in a linear regression appear to be normal.
There are tests of normality but the plot provides and efficient way to inspect a set for normality.

3. (data value, quantile z-score).
The Process
The Normal Probability Plot is a bivariate plot in which we plot
(data value, quantile z-score).
To the quantile z-score:
Double the number of data values (n 2n).
Calculate the z-score for areas of

4. The Process - continued
Order the list of values from least to greatest.
Plot the ordered list of
(data value, quantile z-score)
The Decision:
The data set is NORMAL if the data points in the plot are LINEAR!