Statistics in a Nutshell, 2nd Edition

Chapter 1. Basic Concepts of Measurement

Before you can use statistics to analyze a problem, you must convert information about the problem into data. That is, you must establish or adopt a system of assigning values, most often numbers, to the objects or concepts that are central to the problem in question. This is not an esoteric process but something people do every day. For instance, when you buy something at the store, the price you pay is a measurement: it assigns a number signifying the amount of money that you must pay to buy the item. Similarly, when you step on the bathroom scale in the morning, the number you see is a measurement of your body weight. Depending on where you live, this number may be expressed in either pounds or kilograms, but the principle of assigning a number to a physical quantity (weight) holds true in either case.

Data need not be inherently numeric to be useful in an analysis. For instance, the categories male and female are commonly used in both science and everyday life to classify people, and there is nothing inherently numeric about these two categories. Similarly, we often speak of the colors of objects in broad classes such as red and blue, and there is nothing inherently numeric about these categories either. (Although you could make an argument about different wavelengths of light, it’s not necessary to have this knowledge to classify objects by color.)

This kind of thinking in categories is a completely ordinary, everyday experience, and we ...