Proctor & Gamble $($P&G$)$ conducted research to see if customers who receive free samples of organic laundry soap tend to buy more organic laundry soap. A randomly selected group of P&G customers living in Cincinnati OH were invited to visit the corporate headquarters in town to get samples of organic laundry soap $($i$.$e$.,$ the treatment group$).$ During the four months after providing samples, they recorded how many organic laundry product purchases were made by the treatment group to a randomly selected control group of Kroger shoppers in Cincinnati who buy P&G laundry soap $($based on Kroger Plus member data$).$

After data collection, the P&G Research & Development $($R&D$)$ Team is concerned that relying on people who want free samples to be in the treatment condition may have contaminated results. They also discovered that the customers in the treatment condition received a tour of headquarters when they came to pick up samples, which also raises doubts about the ability to observe the relationship between free samples and purchasing behavior.

P&G R&D Team concerned about?