A researcher wants to test if a new public speaking workshop improves confidence. They advertise the workshop to 100 students. The first 50 students who sign up are placed in the workshop group, while the last 50 to sign up are placed in a control group that receives no workshop. After the workshop, the first group reports significantly higher confidence than the control group. What is the most critical flaw in this study's design that prevents the researcher from concluding that the workshop caused the increase in confidence?