Analyze each of the following reports, explaining the way in which the method of agreement and the method of difference have been jointly applied, and identifying the special force, if any, of their combination.

Scientists have long known that severely restricting the number of calories that mice and other organisms consume lengthens their life span. Animals on low-calorie diets typically have abnormally cool body temperatures. Does low temperature, in itself, result in longer life? The answer is yes.

Bruno Conti, of the Scripps Research Institute in La Jolla, California, genetically engineered mice to have a faulty sense of body temperature. The alteration reduced the animals’ temperatures by 0.03 to 0.05°C below normal; they were given as much food as they wanted, maintaining their normal weight. The low-temperature mice lived about 15 percent longer than normal mice did. 

—Reported in Science, 3 November 2006