Analyze each of the following reports, explaining the ways in which the method of difference has been applied in the investigations recounted. Discuss the strengths and weaknesses of the method of difference as it is used in each case.

The heavy use of salt is widely suspected by experts to be the cause of an epidemic of high blood pressure and many deaths from heart disease around the world. But how to prove that salt is the culprit? There are “natural experiments” in which isolated jungle or farming communities are introduced to modern civilization, move to cities, adopt high-salt diets, and commonly develop high blood pressure. But such evidence is inconclusive because many important factors change together; new stresses and many dietary changes accompany the increase in salt. How can the causal effects of salt by itself be tested?

Dr. Derek Denton, of the University of Melbourne, selected a group of normal chimpanzees, a species biologically very close to humans, in which to conduct the needed trials. A group of chimpanzees in Gabon, with normal blood pressure, were first studied in their natural state. The group was then divided in half, with one half receiving gradually increasing amounts of salt in their diet for twenty months. Normal blood pressure in a chimpanzee is 110/70. In Dr. Denton’s experiment, the animals’ blood pressure commonly rose as high as 150/90, and in some individuals much higher. But among animals in the control group, who received no additional salt, blood pressure did not rise. Six months after the extra salt was withdrawn from their diet, all the chimpanzees in the experimental group had the same low blood pressure they had enjoyed before the experiment. Because there was no other change in the lifestyle of those animals, the investigators concluded that changes in salt consumption caused the changes in blood pressure.
—D. Denton et al., “The Effect of Increased Salt Intake
on Blood Pressure of Chimpanzees,” Nature, October 1995