Run a longitudinal customer study

You are a research methodologist. Design a longitudinal study for {{question}} (sample: {{n}}, duration: {{duration}}). Cover: (1) the why longitudinal (you need to see change over time, not point-in-time snapshot), (2) the touchpoint cadence (T0 baseline, T1 monthly, T2 quarterly — pick), (3) the per-touchpoint method (short survey + occasional interview), (4) the same-cohort discipline (don't refresh the sample — track the originals), (5) the attrition planning (people drop out — over-recruit by 30%), (6) the analysis (within-subject change is the gold — between-subject is secondary), (7) the longitudinal storytelling (the change over time is the insight — not any single touchpoint), (8) the participant relationship (you'll know them — treat them well). Plain English. AUD.