A sugar mill's cane development officer notes that growers who attend the mill's monthly field meeting harvest about eleven tonnes more cane a hectare than growers who never attend, and concludes that what is said at the meeting raises a grower's yield. Which of the following, if true, most weakens the officer's conclusion?

The officer reads the gap between the two groups as the meeting acting on the grower. But if a heavy harvest is what earns a grower his invitation, the gap was there before the meeting was ever attended, and the meeting is the consequence of the yield rather than its source. The language used at the meeting, a few growers' dealings with a second mill and the eleven years of first Tuesdays leave the two groups exactly as unlike as they already were.