A fire officer's note reads: 'Of the calls the station attended last year, the engine reached the spot within the twenty minutes the manual allows in every single case. The station's cover of the district is therefore adequate.' An answer to which of the following questions would be of most use in judging the officer's conclusion?

The officer's record covers the calls that were attended and his conclusion covers the district. Two bodies of fire lie outside his evidence — the calls the station could not take up, and the tracts from which no engine could arrive in time whether a call came or not — and the question asked reaches both, so its answer could either close the gap or blow the conclusion open. The station's strength and the age of its second engine describe the establishment without telling us what it fails to reach; the fuel bill describes what the year cost. A station that attends four calls a year, all of them within two kilometres, would return the officer's perfect record and cover almost nothing.