The constructs studied in quantitative research are referred to as variables. Numbers or values are assigned to variables. A variable can be defined as "a property that takes on different values" (F. Kerlinger, Foundations of Behavioral Research, 1973, p. 29). In other words, it "varies." For example, a demographic variable such as "sex" has two values: male and female.
Each respondent to a survey would have a value of male or female for this variable. For data analysis purposes, one would assign a numeric code to each value (e.g., male = 1; female = 2). Another demographic variable, "age", could take on many possible values. The numeric code assigned to a respondent would simply be his or her reported age. [Alternatively, a survey question on age could have a response set made up of "groupings"; the respondent would check the grouping into which his/her age falls (e.g., 18 - 24 years). In this case, numeric codes would be assigned to each age grouping.]
