Ms. Johnson wants to design a "20% off" coupon for Peninsula Hot Springs Spas customers. This coupon will be available during the month of March at Peninsula Hot Springs locations for all product purchases. Ms. Johnson suggests comparing the sales in March to the sales in April and infer the difference as the causal impact of the coupon on customer sales. As the lead Marketing Research Analyst, provide the following (4 Points total)- 

a. Name the experimental design used in the setup. Represent it in mathematical symbols. (2 Points).

b. What confounding/extraneous variables affect this research design? (2 Points).