define the "case component" as hazard period which is the time period right
before the disease or event onset; and the "control component" as control
period which is a specified time interval other than the hazard period. The
information of the individual "exposed to certain activity or agent during
the hazard period and control period will be compared. If we collect such
information from many subjects who had the disease or event, we can test for
consistent relationships between the disease and the exposure.