Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have an adverse effect on health.[16] It is defined by body mass index (BMI) and further evaluated in terms of fat distribution via the waist–hip ratio and total cardiovascular risk factors.[17][18] BMI is closely related to both percentage body fat and total body fat.[19] In children, a healthy weight varies with age and sex. Obesity in children and adolescents is defined not as an absolute number but in relation to a historical normal group, such that obesity is a BMI greater than the 95th percentile.[20] The reference data on which these percentiles were based date from 1963 to 1994, and thus have not been affected by the recent increases in weight.[21] BMI is defined as the subject's weight divided by the square of their height and is calculated as follows.