Analyze each of the following scientific reports, explaining how the pattern of the method of agreement is manifested by each. Discuss, in each case, the limitations of the method of agreement as applied to that quest for a causal connection.

A large extended family in the town of Cartago, Costa Rica, has long suffered an unusual affliction—an incurable form of genetically caused deafness. Children born into the family have a 50 percent chance of developing the disease, and learn their fate at about the age of ten, when those who have inherited a genetic mutation fi nd that they are beginning to lose their hearing. Scientists from the University of Washington have recently traced the cause of the family’s affliction to a previously unknown gene, named the diaphanous gene, that helps operate the delicate hair cells in the inner ear that respond to sound vibrations.
This gene has a single mutation appearing in the Costa Rican family, whose founder arrived in Cartago from Spain in 1713, and who suffered from this form of deafness—as have half his descendants in the eight generations since. Many in the family remain in Cartago because the family’s hereditary deafness is well known and accepted there. With only a single family to be studied, and thus very few genetic differences to work with, pinpointing the gene took six years. The critical mutation involved just one of the 3,800 chemical letters that constitute the gene’s DNA. —Reported in Science, 14 November 1997