Читать книгу Medical Statistics - David Machin - Страница 34

One can distinguish between interval and ratio scales. In an interval scale, such as body temperature or calendar dates, a difference between two measurements has meaning, but their ratio does not. Consider measuring temperature (in degrees centigrade) then we cannot say that a temperature of 20 °C is twice as hot as a temperature of 10 °C. In a ratio scale, however, such as bodyweight, a 10% increase implies the same weight increase whether expressed in kilogrammes or pounds. The crucial difference is that in a ratio scale, the value of zero has real meaning, whereas in an interval scale, the position of zero is arbitrary.

One difficulty with giving ranks to ordered categorical data is that one cannot assume that the scale is interval. Thus, as we have indicated when discussing ordinal data, one cannot assume that risk of a corn healing for a current smoker, relative to a non‐smoker, is the same as the risk for a previous smoker relative to a non‐smoker. Were Farndon et al. (2013) simply to score the three levels of smoking as 0, 1, 2 in their subsequent analysis, then this would imply in some way the intervals between the levels or scores have equal numerical value.