New streetlights, then crime fell. Case closed?

Question

A city installs bright new streetlights across one neighborhood. The following year, reported crime in that neighborhood falls by 15 percent, and officials credit the lights.

Why is this not yet convincing, and what comparison would strengthen it?

Accepted Answer

Crime may have been falling everywhere. If the whole city's crime rate was already drifting down that year (a secular trend driven by policing, demographics, or the economy), the lit neighborhood would have seen a decline with or without the lights. Comparing the neighborhood only to its own prior year folds the citywide trend into the "effect" of the lights. Other coincidences. A new patrol schedule, a shuttered problem business, or ordinary year-to-year noise in a single area's crime count can each produce a 15 percent swing. What would strengthen it: control areas. Compare the change in th…

New streetlights, then crime fell. Case closed?

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