Note the persistence in the response of sales to price. Consider a simple approach to predicting this series and the implications. The univariate, i.e. auto-projective model is based on the relationship between successive observations. Simplifying the graph, we have the historical values and the predicted values. One of the omitted variables is Price.
Price is a lever to create and to dislocate sales from one period to another.
This dislocation effect creates havoc for univariate modelers. What this means is that if sales were high 4 weeks ago, current sales will be low. Note that the omitted variable Price has a 4 week profile. The dependence of Sales on Price can be seen in a scatterplot showing how Sales increase when the Price is low and Sales decrease at standard pricing. Notice how all the points reflecting low price reflect high volume while all the points for high price indicate low volume.