This graph plots the residuals versus their expected values when the distribution is normal. The residuals from the analysis should be approximately normally distributed. In practice, for data with a large number of observations, moderate departures from normality do not seriously affect the results.

The normal probability plot of the residuals should roughly follow a straight line. Use this plot to look for the following:

If your data have fewer than 50 observations, the plot may display curvature in the tails even if the residuals are normally distributed. As the number of observations decreases, the probability plot may show even greater variation and nonlinearity.

For the glucose data, the residuals exhibit some curvature at the tails. However, there are less than 50 observations in the data so the plot does not necessarily indicate nonnormality.