When using long data, each subject has data in multiple rows. This is because each row represents what?

• A. A single measurement across time
• B. One observation per subject
• C. The subject's repeated measurement over time
• D. A complete dataset for the subject

Answer: (B)

In long data, the row is the unit of observation. Each row captures a single measurement occasion for a subject—at a specific time or condition. Because the same subject is measured multiple times, you end up with several rows for that subject. So the row represents one observation—one measurement instance for that subject. The other options describe broader concepts (a single measurement across time, the entire dataset for a subject, etc.) that aren’t what each row itself represents.