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My book says that ratio levels of measurement is the highest form of measurement and adheres to the same rules as as interval level measurement (distances between intervals of the scale are numerically equal), but it can have an absent property.

The examples are weight, height, blood pressure, pulse, etc.

How can weight or height be a ratio measurement? What thing being measured for weight has not weight? Or has no height? I understand no blood pressure or pulse.

Thanks :)

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Published expositions of "levels of measurement" seem to be of two kinds: cookbooks for people who are being taught how to use it, and fairly abstruse theoretical accounts in obscure journals kept in the sub-sub-sub-sub-basement of the library where no one's gone for a couple of decades. –  Michael Hardy Jan 13 '12 at 0:40
    
Now that I've said that, maybe this is somewhere between those two extremes: "Statistics and the Theory of Measurement" by D. J. Hand, Journal of the Royal Statistical Society. Series A, (Statistics in Society) Vol. 159, No. 3 (1996), pp. 445--492. –  Michael Hardy Jan 14 '12 at 18:07
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If you have 1kg of chocolate and every day you eat 100g then after ten days you have 0g of chocolate, i.e. none left. As importantly, after five days you can meaningfully say you have half the chocolate you had initially.

Now try this with temperature measured in degrees Celsius or Fahrenheit. You cannot meaningfully say that air at $2^\circ$ is twice as hot as air at $1^\circ$, or that air at $0^\circ$ has no heat, since you can cool air well below $0^\circ$. Measure temperature in Kelvin from absolute zero and you can make similar kinds of statements (though the air may have frozen by then).

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