# Normal probability plot

## Assess normality and homoscedasticity assumptions of regression

A normal probability plot is the method by which the

**assumptions of normality and homogeneity of variance are tested in multiple regression**. A normal probability plot is also commonly known as a**P-P plot**. A normal probability plot is a plot of the cumulative frequency of the distribution of standardized residuals yielded from the model against the residuals associated with a normal probability graph scale.* When the plot is a straight line with no curves or outliers, then the assumptions are assumed to be met (**homoscedasticity**). If there are curves along the line or extreme points away from the normal probability graph scale line (outliers), then the assumption has not been met (**heteroscedasticity**).## Hire A Statistician - Statistical Consulting for Students

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*Katz, M.H.

*Multivariable analysis: A practical guide for clinicians and public health researchers.*3rd edn. Cambridge: Cambridge University Press, 2011.