Ankit Vora

Body mass index (BMI) is a measure of body fat based on your weight in relation to your height, and applies to most adult men and women aged 20 and over. Regarding the BMI measure, the World Health Organization (WHO) proposes the following classification:

• BMI <18.5 : Underweight
• BMI [18.5-24.9] : Normal weight
• BMI [25-29.9] : Overweight
• BMI >=30 : Obesity

There is a formula for calculating the BMI measure. The formula is the following:

BMI = weight(kg) / height(metres)²

Thus for the next example, the BMI will be:

weight = 73
height = 1.7
BMI <- weight/height^2
BMI

[1] 25.25952

The function use for calculating the diagnostic is the following:

diagnostic_f <- function(weight, height) {
    BMI_value <- weight/(height^2)
    ifelse(BMI_value < 18.5, "underweight", ifelse(BMI_value < 25, "normal weight", 
        ifelse(BMI_value < 30, "overweight", "obesity")))
}

For our example (weight=73 kg and height=1.30 m) the diagnostic would be:

diagnostic_f(73, 1.7)

[1] "overweight"

The BMI is a relatively easy, cheap and non-invasive method for establishing weight status, and for most people, it correlates reasonably well with their level of body fat.

However, BMI is only a proxy for body fatness. other factors such as fitness, ethnic origin and puberty can alter the relation between BMI and body fatness and must be taken into consideration.