Do the normal linear model assumptions hold?
Although a normal linear model is often used to describe how an explanatory variable, X, affects the distribution of a response, Y, it is not a suitable model for all bivariate data.
Residual plots
The above problems may be evident in a scatterplot of the raw data, but a residual plot often highlights any problems.
In the remainder of this section, we will look in more detail at the four assumptions underlying the normal linear model.