A district health office sent a mobile van for six months to the twelve sub-centres at which attendance in the month of April had been lowest. Attendance at those twelve sub-centres over the six months that followed was on average a quarter higher than it had been in April. An answer to which of the following questions would be of most use in judging whether the van is what raised it?

The twelve were chosen for one bad month, and a bad month is often followed by an ordinary one for reasons that have nothing to do with a van. The only way to tell the van's work from that drift is to look at sub-centres that were equally low and got no van: if they rose too, the van explains nothing. What the van cost bears on whether it is worth running, not on whether it worked; the state of the roads and the van's daily capacity describe the service without telling us what would have happened in its absence.