A researcher wants to consider the effects of preparation on standardized test scores. She intends to compare the scores of students who did and those who did not take a GRE preparation course. She also intends to compare the outcomes of prep classes offered by various test prep companies. The researcher found that students who took a GRE preparation course with Company X and Y scored higher on the GRE than students who did not prepare. But students who took a preparation course with Company Z scored no higher on average than students who did not take a preparation course. These results are an example of