Classification

The Body Mass Index (BMI), also known as the Quetelet index, is a value derived from the weight and height of an individual (both adult men and women). The World Health Organization (WHO) applies the following classification with regard to the BMI result:

• BMI < 18,5 = Underweight
• 18,5 < BMI < 24,9 = Normal weight
• 24,9 < BMI < 29,9 = Overweight
• BMI > 29,9 = Obesity

Calculation

The formula to derive the BMI is as follows: BMI = weight / height^2 With weight in “kg” and height in “m”. To give an example, my BMI will be calculated.

weight <- 75
height <- 1.70
BMI <- weight/(height^2)
BMI

## [1] 25.95156

Diagnosis

Now that the BMI can be calculated, one can be classified in the previously given groups using the following function.

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"))) }

My BMI would result in the following classification:

diagnostic_f(70, 1.70)

## [1] "normal weight"

Conclusion

Thus, the Body Mass Index is an easy and non-invasive way to establish one’s weight status. For most individuals, the BMI correlates quite well with body fatness. Nonetheless, one should not forget that the Body Mass Index is just an appproximation for body fatness. There are other factors (such as fitness, ethnic origin and puberty) which should be taken in account to obtain a more accurate view on one’s body fatness.