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 the results of this study will not apply to customers living outside of Cincinnati, Ohio $($where the corporate headquarters is located$).$ What is the P&G R&D Team concerned about?