A factory issued new ear protectors to the workers at its cutting bench, and over the following year the number of those workers reporting hearing trouble fell by half. The safety officer treats this as proof that the protectors work. Which of the following, if true, casts the most doubt on that inference?

Taking the loudest saws away lowers the noise reaching the bench directly, which would halve the complaints even if the protectors did nothing at all. With two changes made in one year, the fall cannot be laid at the door of either alone. Lighter protectors and hot-weather discomfort concern how the protectors are worn, and hearing trouble elsewhere in the factory says nothing about what changed at this bench.