The measured quantity doesn’t have to be something you’d crack out a ruler to find. It can be anything represented by a number. For example: pH (measured with litmus paper), bone density (measured with a special kind of x-ray) or averages of anything (measured by a formula).
These types of variables are sometimes formally called quantitative variables, especially in academia. The term “measurement variable” is somewhat informal, but (unlike a quantitative variable) they nearly always have a unit attached (e.g. mm, cm, m). You’ll usually come across it in introductory classes or elementary textbooks. There’s no “right” or “wrong” term here though; You can use either one, but if you’re submitting to a journal or writing a thesis, you’ll almost certainly be required to use the term “quantitative variable”.
Types of Measurement Variable
Measurement variables come in a variety of different types:
- Discrete measurement variables: You can count discrete variables and they belong to a finite set. Example: how old you are in years: 21, 23, 59, and so on. Note that the set will have an end (probably 100 or so at most)
- Continuous measurement variables: Continuous variables are not countable, and go on until infinity. The weight of an egg is one example: an egg could weigh 2.01 oz, 2.0031 oz, 2.0000000000002 oz, and so on. Subtypes of continuous variables:
- Interval variables: also continuous, but they have meaningful intervals. As an example, a thermometer might measure in intervals of 0.1 degree.
- Ratio variables: also interval, with a meaningful zero. For example, 0 pounds means that you weigh nothing.
Other broad types of variables
- Nominal (Categorical) variables: can be placed into categories like “under 10s” and “65 or older”.
- Ranked variables: variables that have an order like 1st, 2nd, 3rd…