David L. Johnson wrote:
> On Fri, 02 Dec 2005 06:54:31 -0600, jbuch wrote:
>
>
>>With BMI scaling as Hheight^2 (^ is an easy way to represent the
>>exponent), it is much more realistic than the height^3 or cubic scaling
>>law.
>
>
> why would BMI be more realistic?
>
>
You can find some information on scaling laws for animals here....
1)
http://smccd.net/accounts/goth/other/Life_On_The_Scales.pdf
2)
http://www.primidi.com/2005/02/21.html
and it is a tad technical.
Take a 7 foot person and a 5 foot person.
Would you expect the 5 foot person of "normal build" to have a waist
which is 5/7 of that for a 7 foot tall person?
This linear relationship is what happens if the height^3 (cubed) scaling
law is assumed..... But, if you understood the topic, you would have
known that.
Taking the 34 inch waist for the 7 foot person as "fit physique", would
you expect the waist of the 6 foot person to be 24.3 inches (5/7 of 34
is 24.3 inches) for a "fit physique"?
This is absurd.
Short people of "normal physique" don't have such absurdly small waist
sizes.
If you use the height^2 scaling law, you would expect the waist size
would scale as (5/7)^0.667 (the two thirds power) or you would suspect
that maybe 27.2 inches would be a reasonable waist size for a 5 foot
tall person, rather than 24.3 inches.
The discussion is leading to the suggestion that maybe the waist size
for a "normal physique" 5 foot person would be a little larger than 27.2
inches.....
And if you go into a clothing store that deals in short people's
clothes, you would find a range of waist sizes that you could use to
better calibrate the scaling law that actually fits the body
configurations that humans have.
One suggestion is to use the 3/4 power scaling law (in the second
reference above), which would then give (5/7)^.75 and then a 5 foot
person of "nominal physique" ( with a 7 foot 34 inch waist as "nominal
physique for reference) would have a waist size of 28.0 inches.
"Nominal Physique"
7 foot tall, 34 inch waist
5 foot tall, 28 inch waist (3/4 power scaling law)
5 foot tall, 24.3 inches (linear scaling law, BMI =K * Height^3)
I go with the experts who have studied scaling laws in animals.
BMI as implimented simply sucks and fails to meet what has long been
known about how dimensions of animals scale with size.
The height^(2.2) is an improvement which has the disadvantage of
>
>>being incomprehensible to the average medical or health worker, and
>>incomprehensible to the average "personal trainer" who has no training
>>credentials at all.
>
>
> Units are irrelevant.
>
Anything with matematics is pretty incomprehensible to 95% of the
population. We may not be illiterate, but we have been called
innumerate.... meaning we really screw up stuff based on arithmetical
operations more complex than add, subtract and multiply. Nobody expects
modern youth to actually be able to do long division anymore.... or
count change.
--
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