A town's fire office fitted new hydrants along the market street in March, and reports that the value of goods lost to fire in the twelve months that followed was well below the value lost in the twelve months before. An answer to which of the following questions would be of most use in judging whether the hydrants are what brought the loss down?

Two twelve-month totals hide the one thing that decides the question: whether the fall started when the hydrants went in. If the monthly loss had been dropping since the previous autumn, the hydrants arrived in the middle of something already happening and cannot be credited with it; if it dropped from March and not before, the case is strong. What the hydrants cost bears on value for money, how many were fitted describes the works, and their pattern would matter only if it were the design that were in question.