Pub. date: 2008 | Online Pub. Date: November 27, 2007 | DOI: 10.4135/9781412953948 | Print ISBN: 9781412928168 | Online ISBN: 9781412953948| Publisher:SAGE Publications, Inc.About this encyclopedia
A dummy, or indicator, variable is any variable in a regression equation that takes on a finite number of values so that different categories of a nominal variable can be identified. The term dummy simply relates to the fact that the values taken on by such variables (usually values such as 0 and 1) indicate no meaningful measurement but rather the categories of interest. For example, and The variable X 1 indicates a nominal variable describing ‘treatment group’ (either Treatment A or not Treatment A) and X 2 indicates a nominal variable describing ‘sex.’ The following simple rule always is applied to avoid collinearity and the imposition of a monotonic doseresponse in the regression model: For an exposure with K distinct levels, one level is first chosen as the baseline or reference group. Refer to that level as Level 0, with other K − 1 levels referred to as K ...