A team proposes the theory that chronic sleep restriction increases irritability by impairing executive control. They derive the hypothesis: "Participants restricted to 4 hours sleep per night for 5 nights will show higher irritability scores than participants allowed 8 hours." They then define irritability as the mean score on a validated 10-item self-report scale administered each morning, run the study, and plan an exact-method replication in a new lab. Which step in the sequence below is most directly an example of operationalization?