This is about Stevens’ typology of scales and most of the time it’s a red herring in our change measurement.
Ratio scaling is what it says: being 40 years old really does mean you’ve been out of the womb twice as long as someone who is 20 years old (but see age), weighing 70kg is twice 35kg, a 50 minute session is 5/9ths of a 90 minute group session duration. (Temperature in Kelvin has ratio scaling: 273 degrees Kelvin (zero in Celsius, 32 degrees in Fahrenheit) is twice as hot as 136.5 degrees, but that’s getting us way away from ROM!)
As you can see, only a very small subset of our variables have ratio scaling, though it is quite an important set: duration of session, number of episodes, number of sessions, weight.
But weight illustrates the deeper issues: what a client weighs has different meanings for different people and in different frames of reference. It really only makes sense to compare people’s weights taking into account their age, gender and why we might be measuring it: is this because the client is acknowledging that his/her wish to be even thinner is dangerous and needs to change; is it that the client, epidemiologically correctly, thinks her/his weight is life shorteningly, life restrictingly, too high? In our field, putting numbers to things too often restricts real thinking about the meaning of the thing. Too often that’s used to argue we should abhor numbers but to me that’s as wrong as overvaluing them and using them naively.
Try also #
Stevens’ typology of scales
Chapters 2, 4 and 10?