Statistical Analysis for Education and Psychology Researchers

Figure 8.7b: Fitted least squares

regression line for prediction of

standardized maths ability score from

teacher estimate of maths ability with

95 per cent confidence level for mean

predicted values

Interpretation of Computer Output

The overlay plot shown in Figure 8.1 indicates that the model fits the data reasonably
well, the data points are scattered randomly about the regression line and there are no
obvious outliers. Look for plotted points, #, being some distance from the main cluster of
other points, or for points that are distant from the fitted regression line (plotted by *).
The normal probability plot (see Figure 8.2) indicates that the residuals are reasonably
normal (see Chapter 5 for interpretation of normal probability plots).
The plot of residuals against predicted values shown in Figure 8.3 would ideally have
a random scatter of points around the reference line (mean of zero). There is a suggestion
of non-constant variance in this plot because of the funnel shape indicating that the

Inferences involving continuous data 277