New hygiene protocol in one ward: diff-in-diff

Question

A hospital introduces a new hygiene protocol in Ward A and keeps Ward B, similar in patient mix, as a control. Infection rates before and after:

Ward A (treated): 12% before, 6% after.
Ward B (control): 12% before, 10% after.

Compute the difference-in-differences estimate of the protocol's effect, and interpret the sign given that lower is better.

Accepted Answer

Both wards improved, so the raw drop is too flattering. Ward A's infection rate fell 6 points (from 12% to 6%). But Ward B, without the new protocol, also fell 2 points (from 12% to 10%), probably from a hospital-wide push, seasonal decline in infections, or general awareness. Part of Ward A's improvement would have happened anyway. Difference-in-differences. Subtract the control's change from the treated ward's change: 	ext{DiD} = (6 - 12) - (10 - 12) = (-6) - (-2) = -4. The estimate is negative 4 percentage points. Because the outcome is infections, a negative number is good: the protocol …

New hygiene protocol in one ward: diff-in-diff

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