Like most of the epidemiological studies we need to have control data to determine what "unusual" meant. However, we can�t do the comparison among individuals since everyone is different. Rather, the comparison should be done within the individual. In another words, we will compare the individual�s activity before the disease onset to his/her usual activity. Thus here, we have a "case" component and also a "control" component, but the information of both components will come from the same individual.