Effect of price on sales of a New Zealand wine
Use this page to show that zero slope implies that there is no association between Y and X. Explain that this is an important special case of the model.
The context of the model is the price of a popular New Zealand cabernet sauvignon red wine and its effect on sales in a supermarket chain, measured as a proportion of total red wine sales in a week. The relationship between price and sales will be nonlinear at high prices, but is expected to be reasonably linear within a price range of $12 to $20 per bottle.