Cohort study - strength, directrion, types

Direction of study: 

Exposure to outcome.

Strength of association:  

Estimates risk or rate directly. Strong relation between exposures to outcome.

Types of cohort study:

• Prospective Cohort study – In this study direction of time towards forwards. Study have started before the occurrence of the event and we have to wait and see whether the event will occur or not and in what proportion and in what groups
• Example: Some study started 70-80 years back but still the study has been going on. Such as,  Atom bomb in Japan story: 
• Study group: Paratrooper and soldier involved with bombing and field war who are exposed to nuclear radiation
• Control group: Soldiers from other war
• Still study is going on to see the effect of exposed nuclear radiation
• Retrospective Cohort study- In this study direction of time towards backwards. Study have started after the occurrence of the event but we started from the history of exposure and follow their record and see whether the event was occur in what proportion in what groups?
• Combined Prospective and Retrospective Cohort study – In this study direction of time towards backwards but continue to forward also.