A key aspect of the measurement process involves placing observations of measurements into groups or categories on the basis of unequivocally shared features. Hence a category is a homogeneous grouping of data. For example, the variable ‘sex’ would have two categories, male and female; the variable ‘social class’ might have three categories, upper, middle, and working. In the former case the rule for assigning cases or observations to the appropriate category is relatively simple. In the latter, the rules would be more complex, and dependent upon the class theory being used. See also coding; rules of correspondence.