-----Original Message-----
>From: Peter Flom <peterflomconsult...@mindspring.com>
>Sent: Oct 26, 2009 10:58 AM
>To: medstats@googlegroups.com, MedStats <medstats@googlegroups.com>
>Subject: {MEDSTATS} Ratios as DVs and IVs: BMI (was Re: Interesting results, how to deal with these interaction effects?)
>Thanks to Frank Harrell and Bruce Weaver for their insights and
>recommendations.
>In the article by Kronmal to which Bruce refers below, he argues against any use >of a ratio DV or IV
>This got me thinking about studies that use BMI, either as DV or IV. BMI is, of course,
>a ratio.
>Are these studies therefore bogus?
>Peter
>-----Original Message-----
>>From: Bruce Weaver <bwea...@lakeheadu.ca>
>>Sent: Oct 23, 2009 10:16 AM
>>To: MedStats <medstats@googlegroups.com>
>>Subject: {MEDSTATS} Re: Interesting results, how to deal with these interaction effects?
>>On Oct 23, 9:26 am, Frank Harrell <f.harr...@vanderbilt.edu> wrote:
>>> The laws of arithmetic are violated any time you use % change as a
>>> dependent variable. A simple way to see this is that an increase of
>>> 100% is balanced by a decrease of 50%. Percent changes may only be
>>> computed on group summary measures, not per-patient measures.
>>Peter, here are a couple articles that might help you argue Frank's
>>point about % change with the investigators.
Discussion subject changed to "{MEDSTATS} Re: Ratios as DVs and IVs: BMI (was Re: Interesting results, how to deal with these interaction effects?)" by Juliet Hannah
Any more insights into the BMI question you brought up. It seems like
a good question. BMI
is a well-studied variable, and seems to approximate some measure of
fat (along with waist-hip ratio).
It is expensive to obtain other measures of fat. Maybe in this case,
there is evidence that BMI
approximates the underlying measure (central adiposity) so this makes
it more justifiable?
> I neglected to mention that Kronmal does use BMI as one of his examples of what not to do
> Peter
> -----Original Message-----
>>From: Peter Flom <peterflomconsult...@mindspring.com>
>>Sent: Oct 26, 2009 10:58 AM
>>To: medstats@googlegroups.com, MedStats <medstats@googlegroups.com>
>>Subject: {MEDSTATS} Ratios as DVs and IVs: BMI (was Re: Interesting results, how to deal with these interaction effects?)
>>Thanks to Frank Harrell and Bruce Weaver for their insights and
>>recommendations.
>>In the article by Kronmal to which Bruce refers below, he argues against any use
>>of a ratio DV or IV
>>This got me thinking about studies that use BMI, either as DV or IV. BMI is, of course,
>>a ratio.
>>Are these studies therefore bogus?
>>Peter
>>-----Original Message-----
>>>From: Bruce Weaver <bwea...@lakeheadu.ca>
>>>Sent: Oct 23, 2009 10:16 AM
>>>To: MedStats <medstats@googlegroups.com>
>>>Subject: {MEDSTATS} Re: Interesting results, how to deal with these interaction effects?
>>>On Oct 23, 9:26 am, Frank Harrell <f.harr...@vanderbilt.edu> wrote:
>>>> The laws of arithmetic are violated any time you use % change as a
>>>> dependent variable. A simple way to see this is that an increase of
>>>> 100% is balanced by a decrease of 50%. Percent changes may only be
>>>> computed on group summary measures, not per-patient measures.
>>>Peter, here are a couple articles that might help you argue Frank's
>>>point about % change with the investigators.
I have not had any replies about BMI. I am not very impressed with BMI as a measure, even ignoring the fact that it's a ratio. It's just not a good measure of overweight; it ignores too many factors that vary both across individuals and across groups.
-----Original Message-----
>From: Juliet Hannah <juliet.han...@gmail.com>
>Sent: Nov 1, 2009 11:58 AM
>To: medstats@googlegroups.com
>Subject: {MEDSTATS} Re: Ratios as DVs and IVs: BMI (was Re: Interesting results, how to deal with these interaction effects?)
>Hi Peter,
>Any more insights into the BMI question you brought up. It seems like
>a good question. BMI
>is a well-studied variable, and seems to approximate some measure of
>fat (along with waist-hip ratio).
>It is expensive to obtain other measures of fat. Maybe in this case,
>there is evidence that BMI
>approximates the underlying measure (central adiposity) so this makes
>it more justifiable?
>Juliet
>On Mon, Oct 26, 2009 at 10:01 AM, Peter Flom
><peterflomconsult...@mindspring.com> wrote:
>> I neglected to mention that Kronmal does use BMI as one of his examples of what not to do
>> Peter
>> -----Original Message-----
>>>From: Peter Flom <peterflomconsult...@mindspring.com>
>>>Sent: Oct 26, 2009 10:58 AM
>>>To: medstats@googlegroups.com, MedStats <medstats@googlegroups.com>
>>>Subject: {MEDSTATS} Ratios as DVs and IVs: BMI (was Re: Interesting results, how to deal with these interaction effects?)
>>>Thanks to Frank Harrell and Bruce Weaver for their insights and
>>>recommendations.
>>>In the article by Kronmal to which Bruce refers below, he argues against any use
>>>of a ratio DV or IV
>>>This got me thinking about studies that use BMI, either as DV or IV. BMI is, of course,
>>>a ratio.
>>>Are these studies therefore bogus?
>>>Peter
>>>-----Original Message-----
>>>>From: Bruce Weaver <bwea...@lakeheadu.ca>
>>>>Sent: Oct 23, 2009 10:16 AM
>>>>To: MedStats <medstats@googlegroups.com>
>>>>Subject: {MEDSTATS} Re: Interesting results, how to deal with these interaction effects?
>>>>On Oct 23, 9:26 am, Frank Harrell <f.harr...@vanderbilt.edu> wrote:
>>>>> The laws of arithmetic are violated any time you use % change as a
>>>>> dependent variable. A simple way to see this is that an increase of
>>>>> 100% is balanced by a decrease of 50%. Percent changes may only be
>>>>> computed on group summary measures, not per-patient measures.
>>>>Peter, here are a couple articles that might help you argue Frank's
>>>>point about % change with the investigators.
I agree with Peter. When BMI is discussed as a measure, I think of a forward in a rugby union team, who will have a high BMI despite not carrying much fat. But then I wonder, "What alternative measures are there ?"
----- Original Message ----- From: "Peter Flom" <peterflomconsult...@mindspring.com>
To: <medstats@googlegroups.com>; <medstats@googlegroups.com>
Sent: Sunday, November 01, 2009 8:18 PM
Subject: {MEDSTATS} Re: Ratios as DVs and IVs: BMI (was Re: Interesting results, how to deal with these interaction effects?)
Hi Juliet
I have not had any replies about BMI. I am not very impressed with BMI as a measure, even ignoring the fact that it's a ratio. It's just not a good measure of overweight; it ignores too many factors that vary both across individuals and across groups.
Peter
-----Original Message-----
>From: Juliet Hannah <juliet.han...@gmail.com>
>Sent: Nov 1, 2009 11:58 AM
>To: medstats@googlegroups.com
>Subject: {MEDSTATS} Re: Ratios as DVs and IVs: BMI (was Re: Interesting >results, how to deal with these interaction effects?)
>Hi Peter,
>Any more insights into the BMI question you brought up. It seems like
>a good question. BMI
>is a well-studied variable, and seems to approximate some measure of
>fat (along with waist-hip ratio).
>It is expensive to obtain other measures of fat. Maybe in this case,
>there is evidence that BMI
>approximates the underlying measure (central adiposity) so this makes
>it more justifiable?
>Juliet
>On Mon, Oct 26, 2009 at 10:01 AM, Peter Flom
><peterflomconsult...@mindspring.com> wrote:
>> I neglected to mention that Kronmal does use BMI as one of his examples >> of what not to do
>> Peter
>> -----Original Message-----
>>>From: Peter Flom <peterflomconsult...@mindspring.com>
>>>Sent: Oct 26, 2009 10:58 AM
>>>To: medstats@googlegroups.com, MedStats <medstats@googlegroups.com>
>>>Subject: {MEDSTATS} Ratios as DVs and IVs: BMI (was Re: Interesting >>>results, how to deal with these interaction effects?)
>>>Thanks to Frank Harrell and Bruce Weaver for their insights and
>>>recommendations.
>>>In the article by Kronmal to which Bruce refers below, he argues against >>>any use
>>>of a ratio DV or IV
>>>This got me thinking about studies that use BMI, either as DV or IV. BMI >>>is, of course,
>>>a ratio.
>>>Are these studies therefore bogus?
>>>Peter
>>>-----Original Message-----
>>>>From: Bruce Weaver <bwea...@lakeheadu.ca>
>>>>Sent: Oct 23, 2009 10:16 AM
>>>>To: MedStats <medstats@googlegroups.com>
>>>>Subject: {MEDSTATS} Re: Interesting results, how to deal with these >>>>interaction effects?
>>>>On Oct 23, 9:26 am, Frank Harrell <f.harr...@vanderbilt.edu> wrote:
>>>>> The laws of arithmetic are violated any time you use % change as a
>>>>> dependent variable. A simple way to see this is that an increase of
>>>>> 100% is balanced by a decrease of 50%. Percent changes may only be
>>>>> computed on group summary measures, not per-patient measures.
>>>>Peter, here are a couple articles that might help you argue Frank's
>>>>point about % change with the investigators.
>When BMI is discussed as a measure, I think of a forward >in a rugby union team, who will have a high BMI despite not carrying much >fat. But then I wonder, "What alternative measures are there ?"
Being American, I think of running backs in American football. There was one who was 5'9" and over 200 pounds (1.75 m and 95kg) and not any excess fat.
Better measures? The best simple one is probably waist-hip ratio. Although it, too, is a ratio.
Discussion subject changed to "{MEDSTATS} Re: Ratios as DVs and IVs: BMI (was Re: Interestingresults, how to deal with these interaction effects?)" by John Sorkin
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.
I await the poison arrows that will certainly come my way . . .
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 <peterflomconsult...@mindspring.com> 11/2/2009 9:00 AM >>>
>When BMI is discussed as a measure, I think of a forward >in a rugby union team, who will have a high BMI despite not carrying much >fat. But then I wonder, "What alternative measures are there ?"
Being American, I think of running backs in American football. There was
one who was 5'9" and over 200 pounds (1.75 m and 95kg) and not any excess fat.
Better measures? The best simple one is probably waist-hip ratio. Although it, too,
is a ratio.
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Discussion subject changed to "{MEDSTATS} Re: Ratios as DVs and IVs: BMI (was Re: Interesting results, how to deal with these interaction effects?)" by William Stanbury
The C19 Belgian inventor of the now Anglicised "Body Mass Index" or BMI was
- guess what folks - a research statistician. He spent many years if not
decades ploughing through data and came up with his Quetelet Index, ie. mass
in kg divided by height in metres squared. There were other ratios at the
time, e.g. the Broca Index, yet the Quetelet Index prevailed to become the
BMI. My understanding (to be confirmed or infirmed) of the recent waist-hip
ratio is that it is most applicable to individuals in a given population who
are very obese, in danger of - or needing - surgery, as well as being even
more sensitivethan the BMI to persons of different origins. Such individuals
are comparatively rare in western populations (though perhaps no longer in
anglophone populations...). Clinical trials people are often more focused on
"individuals" and sometimes do not think or work as epidemiologists on
"populations".
Perhaps for now, as with democracy, we should make do with the BMI as the
least bad method?
2009/11/2 Peter Flom <peterflomconsult...@mindspring.com>
> >When BMI is discussed as a measure, I think of a forward
> >in a rugby union team, who will have a high BMI despite not carrying much
> >fat. But then I wonder, "What alternative measures are there ?"
> Being American, I think of running backs in American football. There was
> one who was 5'9" and over 200 pounds (1.75 m and 95kg) and not any excess
> fat.
> Better measures? The best simple one is probably waist-hip ratio.
> Although it, too,
> is a ratio.
Returning to the general, rather than the specific (I realise that there is a lot of the debate about pros and cons of BMI) ....
Whilst I understand the mathematical concerns about analysing a ratio, rather than the constituent components of that ratio, it seems to me that there are often cases in which the ratio (or some other mathematical derivation of two or more observed variables) is the quantity which has practical and meaningful value, in which case it seems (to me) to make sense to analyse that ratio (and little, if any, sense to analyse the components of that ratio). To take a rather extreme illustrative example:
Average speed is the ratio of distance to time. If, say, we had data which consisted of time taken to cover a particular distance (that distance differing between observations), if our interest was in average speed, then it would surely make sense to calculate and analyse that 'ratio'? To my mind, to conduct an analysis in which time and distance were treated separately would be silly and meddlesome, to say the least.
Kind Regards, John
At 11:58 01/11/2009 -0500, Juliet Hannah wrote:
>Hi Peter,
>Any more insights into the BMI question you brought up. It seems like >a good question. BMI >is a well-studied variable, and seems to approximate some measure of >fat (along with waist-hip ratio). >It is expensive to obtain other measures of fat. Maybe in this case, >there is evidence that BMI >approximates the underlying measure (central adiposity) so this makes >it more justifiable?
>Juliet
John
---------------------------------------------------------------- Dr John Whittington, Voice: +44 (0) 1296 730225 Mediscience Services Fax: +44 (0) 1296 738893 Twyford Manor, Twyford, E-mail: Joh...@mediscience.co.uk Buckingham MK18 4EL, UK ----------------------------------------------------------------
I think John Whittington's comments (Nov 02, 2009 below) about not examining the components of a ratio make a lot of sense. Rugby players are not that extreme IMO~they are fit and healthy and one would think perfect specimens for a measure aimed at discriminating between such fit people and others...yet the BMI measure does not seem to do this. To say that this is because they are extreme misunderstands the role of the BMI, IMO; that is to discriminate between fit and healthy people in terms of size, and others. If BMI is expected not to work with these people, for some reason, it should state the conditions under which it is expected to work.
I was surprised at the range of heights. I wondered if the statistician who invented the Quetelet index (later BMI) had included figures like these to validate the index (if he had data for the corresponding weights, and used the ratios as John says). Is the BMI an international index ?
----- Original Message ----- From: "John Whittington" <Joh...@mediscience.co.uk>
To: <medstats@googlegroups.com>
Sent: Monday, November 02, 2009 3:31 PM
Subject: {MEDSTATS} Re: Ratios as DVs and IVs: BMI (was Re: Interesting
results, how to deal with these interaction effects?)
> Returning to the general, rather than the specific (I realise that there > is
> a lot of the debate about pros and cons of BMI) ....
> Whilst I understand the mathematical concerns about analysing a ratio,
> rather than the constituent components of that ratio, it seems to me that
> there are often cases in which the ratio (or some other mathematical
> derivation of two or more observed variables) is the quantity which has
> practical and meaningful value, in which case it seems (to me) to make
> sense to analyse that ratio (and little, if any, sense to analyse the
> components of that ratio). To take a rather extreme illustrative example:
> Average speed is the ratio of distance to time. If, say, we had data > which
> consisted of time taken to cover a particular distance (that distance
> differing between observations), if our interest was in average speed, > then
> it would surely make sense to calculate and analyse that 'ratio'? To my
> mind, to conduct an analysis in which time and distance were treated
> separately would be silly and meddlesome, to say the least.
> Kind Regards,
> John
> At 11:58 01/11/2009 -0500, Juliet Hannah wrote:
>>Hi Peter,
>>Any more insights into the BMI question you brought up. It seems like
>>a good question. BMI
>>is a well-studied variable, and seems to approximate some measure of
>>fat (along with waist-hip ratio).
>>It is expensive to obtain other measures of fat. Maybe in this case,
>>there is evidence that BMI
>>approximates the underlying measure (central adiposity) so this makes
>>it more justifiable?
More even than difference in heights (with which the BMI at least attempts to deal) there could be differences in the relative length of torsos and legs across cultures. I tried to find data on this, and found very little. This would cause cross-cultural problems.
But there are certainly wide variations in this across people, but even that is hard to find data on. On a purely anecdotal level, I am 5'9" (a little shorter than average) but when I buy pants, I usually am at the the low end on inseam length (30" or 31" in jeans).
-----Original Message-----
>From: Martin Holt <m861h...@btinternet.com>
>Sent: Nov 2, 2009 5:14 PM
>To: medstats@googlegroups.com
>Subject: {MEDSTATS} Re: Ratios as DVs and IVs: BMI (was Re: Interesting results, how to deal with these interaction effects?)
>I think John Whittington's comments (Nov 02, 2009 below) about not examining >the components of a ratio make a lot of sense. Rugby players are not that >extreme IMO~they are fit and healthy and one would think perfect specimens >for a measure aimed at discriminating between such fit people and >others...yet the BMI measure does not seem to do this. To say that this is >because they are extreme misunderstands the role of the BMI, IMO; that is to >discriminate between fit and healthy people in terms of size, and others. If >BMI is expected not to work with these people, for some reason, it should >state the conditions under which it is expected to work.
>I was surprised at the range of heights. I wondered if the statistician who >invented the Quetelet index (later BMI) had included figures like these to >validate the index (if he had data for the corresponding weights, and used >the ratios as John says). Is the BMI an international index ?
>Best Wishes,
>Martin Holt
>----- Original Message ----- >From: "John Whittington" <Joh...@mediscience.co.uk>
>To: <medstats@googlegroups.com>
>Sent: Monday, November 02, 2009 3:31 PM
>Subject: {MEDSTATS} Re: Ratios as DVs and IVs: BMI (was Re: Interesting >results, how to deal with these interaction effects?)
>> Returning to the general, rather than the specific (I realise that there >> is
>> a lot of the debate about pros and cons of BMI) ....
>> Whilst I understand the mathematical concerns about analysing a ratio,
>> rather than the constituent components of that ratio, it seems to me that
>> there are often cases in which the ratio (or some other mathematical
>> derivation of two or more observed variables) is the quantity which has
>> practical and meaningful value, in which case it seems (to me) to make
>> sense to analyse that ratio (and little, if any, sense to analyse the
>> components of that ratio). To take a rather extreme illustrative example:
>> Average speed is the ratio of distance to time. If, say, we had data >> which
>> consisted of time taken to cover a particular distance (that distance
>> differing between observations), if our interest was in average speed, >> then
>> it would surely make sense to calculate and analyse that 'ratio'? To my
>> mind, to conduct an analysis in which time and distance were treated
>> separately would be silly and meddlesome, to say the least.
>> Kind Regards,
>> John
>> At 11:58 01/11/2009 -0500, Juliet Hannah wrote:
>>>Hi Peter,
>>>Any more insights into the BMI question you brought up. It seems like
>>>a good question. BMI
>>>is a well-studied variable, and seems to approximate some measure of
>>>fat (along with waist-hip ratio).
>>>It is expensive to obtain other measures of fat. Maybe in this case,
>>>there is evidence that BMI
>>>approximates the underlying measure (central adiposity) so this makes
>>>it more justifiable?
From memory Henri Quetelet worked on the Belgian population in C19, and
possibly also in early C20. He may have worked with data from other cultures
or lands to validate his index however this would need to be checked. This
checking would answer the question whether the BMI was derived from
international data or not and hence whether it is a bona fide international
index. Even if the index is demonstrated not to be a bona fide international
one, Belgium is in a unique population position in Europe at a certain
cultural and economic crossroads. It has been invaded and occupied
historically by many different European cultures, many of whom bred in
Belgium. The land also has a long history of welcoming refugees, whether
political or economic. Furthermore I understand that its capital, Brussels,
overtook New York in 2007 to become the most international city on the
planet. Hence the Belgian population in C19 was already quite heterogeneous
(at the very least by European standards) and is particularly heterogeneous
now by planetary standards. Thus, in addition to the hard, lengthy, and
systematic research work pre-IT performed by Quetelet to conclude upon the
BMI to be the least bad method deduced from his data, perhaps this Belgian
population heterogeneity explains why the Quetelet Index competed well with
other methods at the time to become a planetary "gold standard" - because it
worked best through having been developed from a heterogeneous population?
Nevertheless, if there remain a number of physicians, statisticians, or
epidemiologists on the planet who are not happy with the BMI and are
researching, or about to research, in an endeavour to find a better method,
in addition to cities such as Brussels, Geneva, New York, and Sydney, I
understand that a number of Caribbean islands may also be quite
heterogeneous.
2009/11/3 Peter Flom <peterflomconsult...@mindspring.com>
> More even than difference in heights (with which the BMI at least attempts
> to deal) there could be differences in the relative length of torsos and
> legs across cultures. I tried to find data on this, and found very little.
> This would cause cross-cultural problems.
> But there are certainly wide variations in this across people, but even
> that is hard to find data on. On a purely anecdotal level, I am 5'9" (a
> little shorter than average) but when I buy pants, I usually am at the the
> low end on inseam length (30" or 31" in jeans).
> Peter
> -----Original Message-----
> >From: Martin Holt <m861h...@btinternet.com>
> >Sent: Nov 2, 2009 5:14 PM
> >To: medstats@googlegroups.com
> >Subject: {MEDSTATS} Re: Ratios as DVs and IVs: BMI (was Re: Interesting
> results, how to deal with these interaction effects?)
> >I think John Whittington's comments (Nov 02, 2009 below) about not
> examining
> >the components of a ratio make a lot of sense. Rugby players are not that
> >extreme IMO~they are fit and healthy and one would think perfect specimens
> >for a measure aimed at discriminating between such fit people and
> >others...yet the BMI measure does not seem to do this. To say that this is
> >because they are extreme misunderstands the role of the BMI, IMO; that is
> to
> >discriminate between fit and healthy people in terms of size, and others.
> If
> >BMI is expected not to work with these people, for some reason, it should
> >state the conditions under which it is expected to work.
> >I remembered hearing once that the average Norwegian male was 4" taller
> than
> >a British one ...here I am doing just what John warns against....so I dug
> up
> >a table that gives average heights across nationalities
> >http://en.wikipedia.org/wiki/Human_height#Average_height_around_the_w...
> >I was surprised at the range of heights. I wondered if the statistician
> who
> >invented the Quetelet index (later BMI) had included figures like these to
> >validate the index (if he had data for the corresponding weights, and used
> >the ratios as John says). Is the BMI an international index ?
> >Best Wishes,
> >Martin Holt
> >----- Original Message -----
> >From: "John Whittington" <Joh...@mediscience.co.uk>
> >To: <medstats@googlegroups.com>
> >Sent: Monday, November 02, 2009 3:31 PM
> >Subject: {MEDSTATS} Re: Ratios as DVs and IVs: BMI (was Re: Interesting
> >results, how to deal with these interaction effects?)
> >> Returning to the general, rather than the specific (I realise that there
> >> is
> >> a lot of the debate about pros and cons of BMI) ....
> >> Whilst I understand the mathematical concerns about analysing a ratio,
> >> rather than the constituent components of that ratio, it seems to me
> that
> >> there are often cases in which the ratio (or some other mathematical
> >> derivation of two or more observed variables) is the quantity which has
> >> practical and meaningful value, in which case it seems (to me) to make
> >> sense to analyse that ratio (and little, if any, sense to analyse the
> >> components of that ratio). To take a rather extreme illustrative
> example:
> >> Average speed is the ratio of distance to time. If, say, we had data
> >> which
> >> consisted of time taken to cover a particular distance (that distance
> >> differing between observations), if our interest was in average speed,
> >> then
> >> it would surely make sense to calculate and analyse that 'ratio'? To my
> >> mind, to conduct an analysis in which time and distance were treated
> >> separately would be silly and meddlesome, to say the least.
> >>>Any more insights into the BMI question you brought up. It seems like
> >>>a good question. BMI
> >>>is a well-studied variable, and seems to approximate some measure of
> >>>fat (along with waist-hip ratio).
> >>>It is expensive to obtain other measures of fat. Maybe in this case,
> >>>there is evidence that BMI
> >>>approximates the underlying measure (central adiposity) so this makes
> >>>it more justifiable?
Discussion subject changed to "{MEDSTATS} Re: Ratios as DVs and IVs: BMI (was Re: Interestingresults, how to deal with these interaction effects?)" by Bruce Weaver
On Nov 2, 9:35 am, "John Sorkin" <jsor...@grecc.umaryland.edu> wrote:
> 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.
> I await the poison arrows that will certainly come my way . . .
Hi John. I have no poison arrows. But, in the examples of BMI's
usefulness you provide above, it is used as an explanatory variable.
Kronmal addresses this situation in his article, and argues pretty
persuasively (I think) that one is better off including height and
weight (and possibly their product) as terms in the model. One
argument he offers is that using BMI makes it impossible to see the
relative contributions of height and weight. He gives another example
where there is no association with BMI, but when height & weight are
entered as separate variables, height is strongly associated while
weight is not.
|
