In long format data, how are repeated measures typically stored?

In long format data, how are repeated measures typically stored?

• A. In separate columns for each time point.
• B. In a single row per entity with all measurements collapsed.
• C. In multiple rows with a single measurement per row.
• D. In the same row with predictor columns.

Answer: (C)

Repeated measures in long format are stored as separate observations for each time point, creating a tall, narrow dataset. Each row represents one measurement for a specific subject at a given time, with columns for the subject ID, the time point, and the measured value. If a subject is measured at three time points, there will be three rows for that subject, not one row with multiple time-point columns. This arrangement makes it easy to analyze changes over time and to apply models that expect multiple records per subject. In contrast, the other formats don’t capture repeated measurements across time in the same way: having separate columns for each time point leads to a wide dataset; collapsing all measurements into a single row loses the timing information; and keeping everything in one row with predictors doesn’t reflect multiple observations per subject.

Repeated measures in long format are stored as separate observations for each time point, creating a tall, narrow dataset. Each row represents one measurement for a specific subject at a given time, with columns for the subject ID, the time point, and the measured value. If a subject is measured at three time points, there will be three rows for that subject, not one row with multiple time-point columns. This arrangement makes it easy to analyze changes over time and to apply models that expect multiple records per subject.

In contrast, the other formats don’t capture repeated measurements across time in the same way: having separate columns for each time point leads to a wide dataset; collapsing all measurements into a single row loses the timing information; and keeping everything in one row with predictors doesn’t reflect multiple observations per subject.