A local public school encourages, but does not require, students to wear uniforms. The principal of the school compares the gr
ade point averages of students at this school who wear uniforms with the GPAs of those who do not wear uniforms to determine whether those wearing uniforms tend to have higher GPAs.
This is an observational study. The principal do not actually randomly assign students to either wear or not wear uniforms. Random assignment is essential to conducting a controlled experiment.
