An agriculture office holds that keeping a written account of what a season's crop costs to raise is what keeps a small grower in farming, since of the growers still farming in the block after fifteen years, four in five keep such an account. Which of the following, if true, most strengthens the office's position?

As it stands the office has looked only at growers who lasted, and four in five would prove nothing if the account were equally common among those who did not. Tracing the leavers supplies the missing half, and one in ten against four in five is exactly the gap the claim needs. Free supply of the book, the ruling of its pages and the age of the recommendation describe the scheme rather than its effect, and none of them tells us anything about the growers who left.