Re: {MEDSTATS} Ratios as DVs and IVs: BMI (was Re: Interestingresults, how to deal with these interaction effects?)
Peter Flom
John Sorkin <jso...@grecc.umaryland.edu> wrote
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>It is unfortunate when people besmirch a useful measure on theoretical basis without considering the actual track record of the measure. Yes, there are theoretical reasons for not liking BMI. Two that come quickly to mind are the fact that BMI is a ratio (and in some rare instances one can obtain incorrect inferences when a ratio is used in a model), and second BMI which as a measure of relative obesity will not work well for everyone (whiteness the example of the rugby player of American professional football player). Despite the fact that is does not work for some "odd" people, it works well for a vast segment of the population and is very helpful in studies where it is not feasible to get more direct measures of obesity such as DXA or CT scans. Despite these drawbacks, BMI has proven a very useful measure. When used properly it is a very strong predictor of many outcomes of interest including cardiovascular disease, diabetes, hypertension, stroke, and mortality. BMI is easily and reliably measured (unlike waist-to-hip ratio which can vary widely with the observer especially in obese subjects). Is BMI perfect, absolutely not, but it is useful both clinically and statistically.
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>I await the poison arrows that will certainly come my way . . .
I would not divide BMI (or any measure) into "working" and "not working", rather, I'd say it's completely nonsensical for some people (e.g. athletes) and only partly nonsensical for others. For the very obese, it may be the best available measure.
And I wouldn't say BMI has NO uses; but it is abused drastically. It is abused by insurance companies, who use it to raise premiums or deny coverage to people based on BMI. It is abused by people who take their own BMI and then conclude (perhaps erroneously) that they are fat or not fat. It is also abused by being categorized into (usually) four categories - categorization of a variable like this makes little or no sense. Of course, the categorization is not a problem specific to BMI, but it's done very often with BMI. If a person gains 2 pounds, he shifts from "normal" to "obese" - I think we can all agree this is silly.
Nor are the ridiculous cases confined to professional athletes or others with extreme body types. People vary not just in height, but, at any given height, in torso size, head size, neck length, and so on. A person with a large head, short neck, large torso and short legs (for his or height) will have a higher BMI than a person with the opposite characteristics. I was just speaking the other day with a woman who thought she must be fat because her BMI called her obese, but she wore size 4 jeans and and a waist hip ratio of .75. She wasn't an athlete, although she was a casual runner.
It's true that BMI is easier to collect than other measures - although if we rely on self-report, things can go way off. But is bad information better than no information?
Peter
Peter L. Flom, PhD
Statistical Consultant
Website: www DOT peterflomconsulting DOT com
Writing; http://www.associatedcontent.com/user/582880/peter_flom.html
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John Sorkin
Confuse not the misuse of a measure with the utility of the measure. Yes, BMI is misused, yes expressing BMI as underweight, normal, overweight, obese I, obese II takes a perfectly good continuous variable and replaces it with less than optimum categories, however this is no different than blood pressure, cholesterol concentraion, glucose concentration, or p values. Despite everything, BMI is a useful measure in the vast majority of cases. It can, and will be misused, but that does not meant that knowledgeable people can't use it properly to their advantage, to the advantage of their patient, and public health in general.
John
John David Sorkin M.D., Ph.D.
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
(Phone) 410-605-7119
(Fax) 410-605-7913 (Please call phone number above prior to faxing)
>>> Peter Flom <peterflom...@mindspring.com> 11/2/2009 10:29 AM >>>
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