A researcher in geriatrics designed a prospective study to investigate the effect of two interventions on the frequency of falls. Twenty subjects were randomly assigned to one of the two interventions: education and education plus aerobic exercise training. Three variables considered to be important as control variables were gender; a balance index and a strength index. The higher the balance index, the more stable is the subject and and the higher the strength index, the stronger is the subject. Each subject kept a record on the number of falls during the sixmonths of study