The body mass index (BMI), or Quetelet index, is a measure of relative size based on the mass and height of an individual. The index was devised by Adolphe Quetelet during the course of developing what he called “social physics”, between 1830 and 1850. The BMI for a person is defined as their body mass divided by the square of their height—with the value universally being given in units of kg/m2. So if the weight is in kilograms and the height in metres, the result is immediate, if pounds and inches are used, a conversion factor of 703 (kg/m2)/(lb/in2) must be applied. : The BMI of an individual may also be determined using a table or chart which displays BMI as a function of mass and height using contour lines or colors for different BMI categories, and may use two different units of measurement. There are a wide variety of contexts where the BMI of an individual can be used as a simple method to assess how much the recorded body weight departs from what is healthy or desirable for a person of that height. There is, however, some debate about which values on the BMI scale the thresholds for ‘underweight’, ‘overweight’ and ‘obese’ should be set.