variable

In the physical sciences, variables are the characteristics of entities which are physically manipulated, such as the heat or volume of a substance. In the social sciences, the term refers to attributes which are fixed for each person or other social entity, but which are observed to be at different levels, amounts, or strengths across samples and other aggregate groups. Variables measure a social construct (such as social class, age, or housing type) in a way which renders it amenable to numerical analysis. Thus, the key feature of a variable is that it is capable of reflecting variation within a population, and is not a constant.

There are various difficulties involved in the process of creating variables from constructs (operationalization). The central considerations here are validity (that the variable is a true measurement of the construct it is aimed at) and reliability (that its measurement is reliable).

Variables may be measured at different levels of measurement, but the basic distinction is between continuous variables such as income, and categoric or discrete variables such as class. Relatively few social variables are continuous-forming interval scales such as income or age. Most are discrete, forming ordinal and nominal scales, such as highest educational qualification obtained, or sex, respectively. The different levels of measurement have implications for the types of analysis that can be undertaken.