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.

Adjust the model slope to β1 = -0.03
Sales decreases by a proportion 0.03 (3 percent) for every extra dollar in price. Rotate to a display of density vs y (button at top right). Drag the slider to show how the distn of sales proportion depends on price.
Adjust the model slope to β1 = 0.0
If this model is appropriate, the distribution of sales proportion is the same for all prices.

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.