The body mass index (BMI) uses a formula that includes height and weight. It is often used in research because it requires measurements that are easy to get (height and weight). Because it is easy to measure and does not require a lot of equipment, BMI can be easily assessed for large numbers of people. Still, the BMI has its limitations as a method of assessing body composition and body fatness. Both the BMI and height–weight charts have the problem of classifying some muscular people as overweight when they really are not. This is because people who have a lot of muscle can be high in weight without having too much body fat. This is a good reason for making many assessments for body composition, including measurements such as skinfolds as shown in the Fitness for Life textbooks. These measurements are a more accurate indicator of body fatness than BMI. To review information on BMI and other measurements, refer to lesson 8.1 of the Fitness for Life: Middle School textbook and lesson 13.1 of the Fitness for Life, Sixth Edition high school textbook.