Katherine Is Interested in Designing an Experiment to Examine Whether

Katherine is interested in designing an experiment to examine whether people who use online dating websites are more likely to find satisfying relationships than individuals who do not use such websites. To do so, she designs a study where people in the experimental condition are those who have used online dating websites to find their current romantic partner. Participants in the control condition are individuals who met their partners in person. She measures and compares their current level of relationship satisfaction to reveal that participants who met online are more satisfied than those who met face-to-face. Imagine that you are a journal editor who received her manuscript claiming that online dating causes more satisfying relationship. What is the fatal flaw in her experimental design that would lead you to reject this paper for publication?