Longitudinal Data / Longitudinal Study

**What it means:** Longitudinal data is collected from the **same subjects repeatedly over time** — like taking photos of the same tree every year to watch it grow. This is different from a cross-sectional study, which looks at different subjects at a single point in time. **Why it matters for MHS:** The Volare Study and Boston Children's assessments are longitudinal — they're tracking MHS patients over months and years. This is essential because: - It reveals how symptoms change as patients age - It identifies which measures are most sensitive to change (important for future trials) - It provides a natural history baseline against which treatment effects can be measured - It helps identify "critical windows" where intervention might be most effective