Between 1948 and 1951, researchers asked Framingham residents about their health and lifestyles and measured their blood pressure and weight. They assessed weight using the body mass index (BMI). (BMI is a person's weight in kilograms divided by the square of his or her height in meters. People with BMIs of 25 to 30 mg/kg 2 are overweight, and people with BMIs of 30 kg/m2 or greater are obese.) The researchers followed Framingham residents regularly for many years. By 1990, most residents who had been middle-aged in 1950 had died. The researchers then compared how long the obese, overweight, and normal-weight people had lived.