Statistically informed ideas on how to make the world work better.

Month: January, 2013

Obesity and mortality: challenging the conventional wisdom, part III: BMI is nearly useless

I’ve just written a couple posts critiquing the recent study in the Journal of the American Medical Association claiming that overweight people are at lower risk of mortality. This is the third and last, and will be a detailed exploration of why BMI is a bad metric of obesity.

I am certainly not the first to claim this, and indeed there are many researchers who have been defending measures such as waist circumfrence for years. But my point is not just to trash BMI, nor to defend another measure in particular (I think waist circumfrence is also a red herring), but rather to use BMI as an example of how shoddy thinking can lead to the adoption of poor metrics and thus serious public health consequences.

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Obesity and mortality: challenging the conventional wisdom, part II: Categorisation is fatal

This post is the second in a series critiquing the much-publicized recent study by Flegal et al. in JAMA, which claims that there is a lower risk of mortality among overweight people than normal weight people. As in my last post, my main beef is that our standard categories of BMI are a problem, but here I argue against categorizing altogether. In fact, I think there is a good argument that researchers categorizing BMI is literally fatal for many people.

How so? Well, once we put someone in a bin such as “obese” or “normal,” we have a mental tendency to accept the categories as something real, something objective. But they are not. These cut-offs are arbitrary, chosen mostly because they are nice round numbers. This can be seen easily on a histogram of 53,000 BMIs taken from the NHANES study (American adults age 20+, roughly 1998-2010):

NHANES BMI dist Read the rest of this entry »

Obesity and mortality: challenging the conventional wisdom, part I

A recent article in the Journal of the American Medical Association has gotten a lot of attention for claiming that risk of mortality is lower in overweight people than normal weight people. I will write a series of posts critiquing this article, which I think demonstrates many of the problems with modern epidemiology and research in this field.

This first post is a very simple one: We do not correctly classify obesity. The standard classification used by the article is as follows:

BMI < 18.5 : Underweight

18.5 < BMI < 25 : Normal weight

25 < BMI < 30 : Overweight

30 < BMI < 35 : Obese (grade 1)

35 < BMI : Obese (grades 2-3)

I just calculated my BMI for the first time. BMI is calculated as mass (kg) / height×height (m). I weigh 175-180 lbs (80 kg) and am 6 feet (184 cm) tall. My BMI is thus  around 23.6 – 24.4, depending on the day. This puts me at the upper end of “normal.” In other words, anyone a bit heavier than me is overweight. Here I am:


While I am not underweight (at least not much), anyone who knows me will tell you I am a bit on the lean/slender side. I have to work to avoid being too skinny. I am not at the upper end of normal, I am at the lower end of normal – a pretty broad range on this scale.

Everyone who works with BMI admits that it is an imperfect measure – that it can classify body builders in heavier categories, for example, and that it doesn’t really incorporate body type. But I’m not a body builder, and I have a standard body type, and the standard scale is telling me that I’m not far from being overweight. According to the scale, I would have to lose 55 lbs (25 kg) to become underweight! Even when I had cancer and became a bit emaciated (to the consternation of everyone who saw me), I only lost about 20 lbs.

I use myself as an example, but I am convinced the standard cut-offs are wrong. Many people who are underweight (anorexic or wasting away due to cancer, and thus at higher risk of mortality) are in the “normal” category. And many people at the perfect weight are in the “overweight” category. So it’s no wonder we will see strange results like that from the JAMA study.

How about you? Is your BMI about right according to the scale above?

Oops, erratum…

Well, I’ve just made my first major blunder as a blogger: posting something I immediately need to retract. My last post, on how to use the threat of giving Republican districts what they’ve asked for in order to break the debt ceiling impasse, was completely off-base. Here’s why:

First, it would be completely unconstitutional, given the equal protection clause of the 14th amendment. This states that “no state shall … deny to any person within its jurisdiction the equal protection of the laws.” So there is no credible way to threaten to withhold benefits just from districts that vote a certain way.

Second, and perhaps more importantly, it would set an awful precedent. Even though in this case (as I believe) it is not a raw political threat, but is actually giving voters what they asked for jurisdiction by jurisdiction, the same principle, if legal, could easily be applied to much more raw political threats, such as to withhold all benefits of certain types from districts held by the opposition as a way to compel people to vote for the majority party. This sort of practice would fundamentally corrupt the functioning of a democratic society.

So, as much fun as the idea was for the debt ceiling crisis, in all practical terms the suggestion was way off base. Thanks to my father for pointing this out.

Idea: How to get Republicans to raise the debt ceiling without a compromise

Much has been written about the debt ceiling, particularly in the summer of 2011 when we had the last showdown. It looks like another is looming. Briefly, the US has already authorized lots of spending on things like Social Security benefits, government contracts, etc. This spending exceeds the amount of money being taken in through taxes, etc. This means that the US needs to borrow money to cover what it has agreed to do. However, in order to borrow more money, congress needs to separately authorize an increase in the total amount of debt that can be taken on. And Republicans, not wanting to have more debt, don’t want to raise the debt ceiling this way.

This is as if you signed a contract for renovations on your house knowing you’d have to borrow money for it, but after the work is done you tell the contractor you can’t pay because you’d have to go too far into debt. You shouldn’t have done the renovations to begin with. The Republican goal is to use the threat of defaulting on our obligations to force cuts in spending they want. It seems to me like blackmail – holding the credit rating and the moral reputation of the country hostage to their political goals. And here is how to stop them:

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