Analyze each of the following investigations, or arguments, and indicate which of the methods of causal reasoning—Mill’s methods—are being used in each of them:

The mood changes that many people experience during the shorter days of winter have a physiological basis in the brain, according to a study reported in the British medical journal, The Lancet, in January 2003. One hundred healthy volunteers, ages 18 to 79, allowed researchers to draw blood samples, at different times of the year, from their jugular veins, to get blood as close to the brain as possible. The researchers then correlated levels of brain chemicals, especially serotonin, with the weather data—temperature, air pressure, rainfall, and sunlight—at the times of blood collection. Only sunlight had causal impact; serotonin levels were found to be lowest in the three months of winter, but varied depending on the brightness of the day. “Our findings [the researchers wrote] are further evidence for the notion that changes in release of serotonin by the brain underlie mood seasonality and seasonal affective disorder.”