g what constitutes a healthy or normal bmi? what are some of the reasons it is useful to have a single number, such as a bmi, to describe a height/weight relationship? what are the drawbacks?

BMI is the shortened form for Body Mass Index.

The Body Mass Index is a simple arithmetic for determining one's body fat using their height and weight.    

The formula is given as

or

If an adults BMI falls within the 18.5 to 24.9 range, it is considered healthy.

One of the benefits of using BMI as a health metric is that it is so easy to compute.

Its major disadvantage is that it only accounts for height and weight which can be altered by a variety of circumstances and as such in most cases are not the best indicators of whether or not, one if healthy.

For example, a higher muscle mass will mean more weight and as such, a higher BMI. Generally speaking, more muscle and less fat is healthy. So BMI, in this case, is useless as a metric of health status.

Other factors which alter the reading of BMI are: Gender, Frame Size, Age,

Pregnancy etc.

An individual who is very muscular, pregnant, or have a large frame, will most likely have a higher BMI calculation which may indicate that one is obese or overweight when they're not.

Cheers!