Study design

===Lecture Details===
Robin Bell; Week 5 MED1022; Epi
===Lecture Content===
Exposure is a factor that changes the likelihood of acquiring disease, outcome is the development of disease. 
Descriptive studies are used to engineer a hypothesis, lowest level of evidence. They can be the first observation of a rare occurrence. Ecological studies are done on groups, affected by ecological fallacy where exposure and outcome may not be related at all. There is no way to differentiate between individual cases.
Cross sectional studies measure exposure and outcome at the same time so it is impossible to differentiate whether exposure comes before the outcome. Backwards direction case-control studies look at the cases and controls determined by disease state and move back in time to find a link between exposure and outcome. New cases should be used rather than prevalent cases. An appropriate control group is difficult to find- should be near identical in all but disease. Case-control cannot be used to estimate incidence as disease is already present.
Odds ratio: odds of exposure in cases/ odds of exposure in controls
Cohort study moves forward in time, monitored to see who develops a disease from exposure. Outcome can be reported in risk ratio (relative risk) of exposed compared to unexposed. Nested case-control is case control within a cohort study.
RCT are ultimate study design for causation, there is random allocation and allocation concealment. Cluster randomised trials are where unit of randomisation is not an individual.
Systematic reviews are literature reviews focused on a specific question to bring information together such as RCT to contribute to evidence based medicine. Meta-analysis combines studies that address a hypothesis. 
Errors are biological variation, sampling error from small sample size, measurement error such as abnormal responses from questionnaires. Systematic error includes bias such as non-representative sample size, or recall bias in case-control studies. Confounding is possible in all except RCT. Can be minimised by stratification, randomisation.
===Readings===