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Thread: Bad math in national newspaper.

  1. #1 Bad math in national newspaper. 
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    Last week the UK Independent newspaper, what used to be known as a broadsheet, printed an article about the Body Mass Index, the means by which you determine whether or not you are overweight or underweight. The article is by "Maxine Frith, Social Affairs Correspondent".

    Here is the opening paragraph:
    What is Body Mass Index?

    It is a calculation to determine the amount of fat in a person's body, and therefore their risk of weight-related diseases such as diabetes. The Body Mass Index was devised in 1840 by Adolphe Quetelet, a Belgian scientist, and has since been adopted worldwide as the standard way to measure obesity. To find your BMI, divide your weight in kilograms by your height in metres squared.
    I do hate to quibble, but I don't have a height in "metres squared". My height is in metres. What she meant to say was "..divide your weight in kilogrammes by the square of your height in metres." (If there had been a comma before the "squared" it might have been almost acceptable.)

    Further on, it discusses Juvenile BMI which is slightly different (for adults the thresholds are fixed, for juveniles they are population dependent.)

    People with a BMI of more than 40 are considered to be morbidly obese. For children, the rating is more complex. Juvenile BMI is calculated in the same way, but is compared against the total range of scores for all children of the same age. A child whose BMI falls below the 85th centile of their age group is underweight, while those in the 85th to 95th centiles are classified as overweight and those in the 95th centile or above are obese.

    A study for the Department of Health found that, by BMI status, 43 per cent of men and 33 per cent of women in England are overweight. A further 22 per cent of men and 23 per cent of women are classed as obese.
    That last paragraph contains meaningful numbers. But she goes on:
    Among children, 643,515 boys and 613,048 girls are overweight - about 1 in 10 of the juvenile population. A further 746,662 boys and 675,983 girls are classed as obese - around five per cent of youngsters.
    You know, just typing that out has pointed out to me a different problem with what was written than I had intended to point out: ie the fact that if 1.25m is 1 in 10 (10%), then how can 1.42m be only 5%? What I wanted to point out, however, leaving those bizarre numbers aside for a moment, was the fact that the final results indicating that 10% of youngsters were overweight and 5% were obese, was kind of anticipated since "overweight" was defined as "being fatter than 85% of the population and thinner than 5%" and obese was defined as "being fatter than 95% or the population". In other words you can't ever find a different number than 10% overweight and 5% obese!

    If I'm completely wrong on this or anything else, please do point it out to me. In which case the issue is how do you write something so badly that someone who considers himself relatively scientifically savvy would misunderstand?

    (this is a repost of a thread I created in Pseudoscience, but which garnered no replies - since it's a mathematical question I have moved it here and deleted it from Pseudoscience, where it was called "Unscience in the media".)


    "It is comparatively easy to make clever guesses; indeed there are theorems, like 'Goldbach's Theorem' which have never been proved and which any fool could have guessed." G.H. Hardy, Fourier Series, 1943
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  3. #2  
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    WHich just goes to prove 'you shouldn't believe everything you read in the papers', '95% of statistics are made up on the spot' and of course
    ... dammit tea's ready..


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  4. #3  
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    Bad Math take 2. The Daily Telegraph has as its front page headline
    The case for speed cameras destroyed in a flash.

    Speeding causes only 5pc of crashes, official figures reveal.
    Spot the fallacy there, anyone? Of course speeding only accounts for a small number of car crashes, but the speed cameras are trying to cut down on serious injury and death due to car crashes, and I'm pretty sure you'll find that the incidents that cause injury and death and the speeding incidents have a far greater correlation.
    "It is comparatively easy to make clever guesses; indeed there are theorems, like 'Goldbach's Theorem' which have never been proved and which any fool could have guessed." G.H. Hardy, Fourier Series, 1943
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