A food inspector's note reads: 'Since the new testing order came into force, seizures of adulterated cooking oil in the district have gone up fourfold. Adulteration of cooking oil in the district must therefore be spreading.' An answer to which of the following questions would be of most use in judging the inspector's conclusion?

A seizure requires both adulterated cooking oil and somebody looking for it, so a fourfold rise in seizures is exactly what more adulteration would produce and exactly what four times as much searching would produce over an unchanged trade. Knowing how much searching was done on each side of the order separates the two, which is what the inspector's figure by itself cannot do. The penalty bears on what a convicted trader suffers; where the cooking oil is made bears on where any adulteration would begin; and the officer competent to seize bears on the procedure. None of the three tells us whether the district's cooking oil changed or only the district's vigilance.