The quantitative independent variables used in regression equations, which usually take values over
some continuous range. Frequently, one may wish to include the quality independent variables, often
called dummy variables, in the regression model in order to (i) capture the presence or absence of a
‘quality’, such as male or female, poor or rich, urban or rural areas, college degree or do not college
degree, different stages of development, different period of time; (ii) to capture the interaction
between them; and, (iii) or to take on one or more distinct values.