Observational studies

Observational studies

1. Cohort study

   • Definition
     • “group of people with defined characteristics who are followed up to determine incidence of, or mortality from, some specific disease, all causes of death, or some other outcome.”
   • Study Design
     • outcome or disease-free study population is first identified by the exposure or event of interest and followed in time until the disease or outcome of interest occurs
       • Because exposure is identified before the outcome
     • Can be prospective or retrospective
       • Prospective studies are carried out from the present time into the future.
         • collect specific exposure data and may be more complete
         • Long follow-up period while waiting for events
       • Retrospective cohort studies are carried out at the present time and look to the past to examine medical events or outcomes.
         • less costly and shorter than prospective cohort studies
         • limited control the investigator has over data collection
   • Methodological Issues
     • Selection of Subjects
       • A critical characteristic of subject selection is to have both the exposed and unexposed groups be selected from the same source population
     • Attrition Bias
       • Loss to follow-up is a situation in which the investigator loses contact with the subject, resulting in missing data. If too many subjects are loss to follow-up, the internal validity of the study is reduced. loss to follow-up rate not exceed 20%

2. Case-control studies

   • Defintiion
     • Case-control studies were historically borne out of interest in disease etiology. The diseased patient is questioned and examined, and elements from this history taking are knitted together to reveal characteristics or factors that predisposed the patient to the disease.
   • Study design
     • subjects by outcome status at the outset of the investigation. Outcomes of interest may be whether the subject has undergone a specific type of surgery, experienced a complication, or is diagnosed with a disease
       • Data about exposure to a risk factor or several risk factors are then collected retrospectively, typically by interview, abstraction from records, or survey. Case-control studies are well suited to investigate rare outcomes or outcomes with a long latency period because subjects are selected from the outset by their outcome status.
   • Methodological Issues
     • Selection of Cases
       • it is imperative that the investigator has explicitly defined inclusion and exclusion criteria prior to the selection of cases.
     • Multiple Controls
       • Investigations examining rare outcomes may have a limited number of cases to select from, whereas the source population from which controls can be selected is much larger.
     • Selection of Controls
       • An important principle is that the distribution of exposure should be the same among cases and controls; in other words, both cases and controls should stem from the same source population
     • Matching
       • Each case is typically individually paired with a control subject with respect to the background variables. The exposure to the risk factor of interest is then compared between the cases and the controls. This matching strategy is called individual matching. Age, sex, and race are often used to match cases and controls because they are typically strong confounders of disease.
     • Bias in Case-Control Studies
       • Evaluating exposure status can be the Achilles heel of case-control studies. Because information about exposure is typically collected by selfreport, interview, or from recorded information, it is susceptible to recall bias, interviewer bias, or will rely on the completeness or accuracy of recorded information, respectively.