A psychologist conducts a study and finds that cities with a higher number of public parks also have lower reported rates of anxiety among their residents. The psychologist concludes that building more public parks will cause a decrease in urban anxiety. What is the primary flaw in this conclusion?

A researcher wants to test if a new energy drink improves memory. They ask for volunteers from a large lecture hall. The first 50 students who sign up are assigned to the 'energy drink' group, and the next 50 students are assigned to the 'no drink' control group. The 'energy drink' group performs significantly better on a subsequent memory test. The researcher concludes the drink enhances memory. What is the most significant flaw in this research design that challenges the conclusion?