A salt-pan cooperative holds that the wind pump it fitted last season is what raised its harvest of salt, the harvest having risen by a sixth. Which of the following, if true, most strengthens the cooperative's position?

The chief doubt about any such claim is that the year it is measured from was a poor one, so that the rise is a return to normal. Six steady seasons with the base season sitting in the middle of them close that doubt, and ordinary rain and working days close the two other ways the harvest might have risen on its own. The pump's supplier, how the families are paid and the pans' distance from the sea neither rule anything out nor make the base year typical.