In market research, a variable is a value that changes as a result of direct intervention or based on changes in another variable. The former is referred to as independent, while the latter is called dependent variable. An independent variable is an input that influences the dependent ones. For example, the four elements of the marketing mix - product, price, place, promotion - can be considered independent variables, as any change in their value will influence the dependent variables. To test more accurately the effects of a change, you should adjust only one independent variable at a time.