Study Design: Experimentation, Observation, Surveying

If the goal of a study is to demonstrate a cause-and-effect relationship, then the ideal approach is a designed experiment. The hallmark feature of an experiment is that the investigator controls and manipulates the values of one or more variables and then observes changes in the response variable. For example, in the concrete industry engineers might want to know how varying the amount of different additives affects the strength of the concrete. A research team would plan an experiment in which they would systematically vary specific additives and conditions, and then measure the strength of the resulting batch of concrete.

Similarly, consider a large retail company that has a "customer loyalty" program, offering discounts to its regular customers who present their bar-coded key tags at the checkout counter. Suppose the firm wants to nudge customers to return to their stores more frequently, and generates discount coupons that can be redeemed if the customer visits the store again within so many days. The marketing analysts in the company could design an experiment in which they vary the size of the discount and the expiration date of the offer, and then see when customers return.