Weights of pairs of adult brothers

Use this diagram to explain that the least squares line for predicting Y from X is different from the least squares line for predicting X from Y.

Firstly click the checkbox Older brother to see the residuals for predicting the older brother's weight from that of the younger brother. Drag the red arrow to make the residuals small.

Now uncheck this checkbox and click Younger brother to see the residuals for predicting the younger brother's weight. These are horizontal lines in the scatterplot. Observe that the residuals are fairly large and drag the line to make them smaller.

Finally, set both checkboxes and drag the line. The two bars representing the two kinds of residual sum of squares show that different lines minimising the two residual sums of squares.

The data are artificial.