Not all BMIs are the same
BMI, or the equivalent ‘acceptable’ weight, can vary quite a lot, depending on who, when, where & how it is calculated
In this country we tend to think of our height in feet & inches, our weight in stones. BMI needs meters & kilogrammes if it is to be checked against the familiar 20, 25, 30 yardstick
So conversion is required
In every day life we tend to think that 1 inch = 2½ centimetres, which is perfectly adequate for most purposes. But it makes you out shorter than you really are – the correct conversion factor should be 2.54
Then there is the question of rounding. Do we carry all the decimal points through at every stage of the calculation, or round, or truncate to meaningful ‘real’ numbers at certain points
Using my own height as an example (5ft 10in) I might be branded obese at 12 stone, or not until I get to nearer 12½
At the level of the individual it may not matter too much – whatever the detail, I am getting to be very heavy
Actually I would start to get a bit worried if I got near to 11 stone, more than I have ever been in my life before
But in any box ticking, performance monitoring or statistical exercise comparing populations or changes in a single population over time, or calibrating the relationship between BMI & health, these biases could matter
