The concept of time is very important in determining the type of epidemiologic study to be done.  If you are looking for the relationship of cause and effect during present time,  you will need a cross-sectional study.  However, if you are looking at the cause and effect relationship of the past, you will need a retrospective study.  If you looking at the relationships in the future, you will need a prospective study. I will be lecturing on each one of these studies more in details presenting with the aid of the next few slides.