For the data on house sales shown partly in Table 9.5 on page 268, the number of bedrooms has a moderately strong positive correlation with selling price.

Controlling for size of home, however, this association diminishes greatly.

(a) Explain how this could happen, illustrating with a diagram showing potential direct and indirect effects of number of bedrooms on selling price.

(b) Explain what it means to say that there is confounding in the effects of size of home and number of bedrooms in their effects on the selling price.