Two made-up clinics report how many patients recovered within a week after a small operation…

Step 1. Groups. X: $95\%$ under $60$ and $60\%$ over $60$. Y: $90\%$ under $60$ and $55\%$ over $60$.

Step 2. Overall. X: $\dfrac{95 + 240}{500} = 67\%$. Y: $\dfrac{360 + 55}{500} = 83\%$.

What it means. For a patient over $60$, clinic X has the better record, $60\%$ against $55\%$. Clinic Y looks better overall only because most of its patients are younger, and younger patients recover more often anywhere. Suggest clinic X, and say why.