Because the test variables are assumed to be ordinal, the Mann-Whitney and Wilcoxon tests are based on ranks of the original values and not on the values themselves.

  • The rank table is divided into three panels, one panel for each test variable.
  • The first test variable, Travel ADL, measures the ability to regularly get around the community. It ranges from 0 to 4, where 0 = Same as before illness and 4 = Bedridden. All 46 women in the control group and all 54 women in the treatment group provided valid data for this variable.
  • First, each case is ranked without regard to group membership. Cases tied on a particular value receive the average rank for that value. After ranking the cases, the ranks are summed within groups.
  • Average ranks adjust for differences in the number of patients in both groups. If the groups are only randomly different, the average ranks should be about equal. For Travel ADL, the average ranks are over 9 points apart.
  • The test variables Cooking ADL and Housekeeping ADL contain missing data. For these variables, the value 4 = Never did any; thus, these scales do not apply to all patients.
  • However, for those to whom they do apply, there are differences of about 12 to 13 points between the average ranks of the treatment and control groups.