What is a QQ plot?
A QQ plot (quantile-quantile plot) is a graphical tool that compares the empirical quantiles of a variable (or residuals) with the theoretical quantiles of a reference distribution (usually normal). The QQ plot can help us assess if the variable (or residuals) follows a certain distribution, by checking if the points fall approximately on a straight line. If there are deviations from linearity, such as curvature or outliers, it indicates that the variable (or residuals) does not follow that distribution.
How to use qqplot() instead of qqPlot() in car package?
In this article, we will explain how to use the base R function ‘qqplot()’ instead of the ‘qqPlot()’ function from the `car` package to check the normality of a variable or a set of residuals. I will also show how to customize the plot and add confidence envelopes.