you believe changes in the sales are caused by changes in price and advertising expenditures (explanatory variables).
Let’s consider the data shown in Table 1. First, we’ll examine our assumption that changes in price cause changes in sales. Figures 2a and 2b seem to support the causal relationship: sales fall as prices rise and vice versa. Ignore for the moment the dashed line. Figure 2b suggests that the relation between price and sales is linear.