Observational Studies Essay

Observational studies are studies in which the researcher merely observes the study units, making one or more measurements on each. (R.C Elston &W.D Johnson, 2008)
Observational studies can be prospective or retrospective. In some cases, studies are combined and thus have elements of both prospective and retrospective studies (such as when exposure has happened but subjects are followed to observe the outcomes of the exposure).
A prospective study is a study design that studies subjects going forward in time. That is, researchers group subjects based on their exposure (to either risk or protective factors) and follow the impact of the exposure over time. In other words, investigators recruit subjects and gather study data going forward (as

The four main types of observational studies are cross-sectional, case-control, cohort, and ecological

Advantages
Less time-consuming than case-control or cohort studies, good, quick picture of prevalence of exposure and prevalence of outcome.
Disadvantages.
Difficult to determine temporal relationship between exposure and outcome (lacks time element), May have excess prevalence from long duration cases (such as cases that last longer than usual but may not be serious), expensive.
2. Case-control
Examines multiple exposures in relation to an outcome; subjects are defined as cases and controls, and exposure histories are compared.
Advantages
Relatively inexpensive, Less time-consuming than cohort studies, Can evaluate effects of multiple exposures, Efficient for rare outcomes or outcomes with long induction or latency periods.
Disadvantages
Subject to recall bias (based on subjects’ memory and reports), Inefficient for rare exposures, Difficult to establish clear chronology of exposure and outcome

3. Cohort (specifically prospective)
Examines multiple health effects of an exposure; subjects are defined according to their exposure levels and followed over time for outcome occurrence.