Normal Probability Plot Jim Bohan Manheim Township School District Lancaster PA jim_bohan@mtwp.k12.pa.us

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.

(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

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!