The marketing manager of a well-known automobile engine additive suspects that the use of an in-store display affects the price elasticity of his product. Specifically, he suspects that the presence of an in-store display increases the product's price elasticity relative to no in-store display. To test this hunch, he would like to do a sales experiment with the 654 retail stores that carry this product. Currently the additive is being sold for $7.99 a bottle.

Specify the variables and groups that would describe the experiment that this manager could use to test this hunch. Explain your choices.