In real life
The National Family Health Survey (NFHS) visits hundreds of thousands of Indian households and records, at one point in time, children's heights, women's haemoglobin, vaccination status and much more. It tells us, for example, that 29.3% of children under five were stunted in NFHS‑6 (2023–24). That's a cross‑sectional study, and it guides national nutrition policy.