Mary wants to determine if there is a relationship between room color and sleep. Based on her research, Mary makes an educated guess that people fall asleep more quickly in a room painted blue than in a room painted yellow. She asks several people which color they like better—yellow or blue—and uses their responses to determine if her educated guess was correct. What can you infer is missing from Mary's scientific investigation?

A testable hypothesis that is based on research
An experiment that directly tests the hypothesis
Variables to be tested by an experiment
Research on the topic to see what others have learned