In contrast to anorexia nervosa and bulimia nervosa, obesity is classified not as a psychiatric disorder but as a medical disorder. Obesity is an excessive accumulation of body fat and operationally is defined as overweight. The BMI, defined as weight (kg)/height (m²), has the highest correlation, 0.8, with body fat measured by other, more precise laboratory methods. Mild overweight is defined as a BMI of 25–30 or body weight between the upper limit of normal and 20% above that limit on standard height–weight charts. Obesity is defined as a BMI greater than 30 or body weight greater than 20% above the upper limit for height (Bray 1978; see Figure 23–1).