Epidemiological Studies

Epidemiological Studies

1. Observational

   • —Selected Units: individuals, groups —Study Populations: cross-sectional, longitudinal —Data collection timing: prospectively, retrospectively, combination —Data collection types: primary, secondary ------ Types of Epidemiologic Observational Studies are based on: —type of sampling from population: —Exposure —Disease —Neither —temporal sequence of observation —One time point   —Forward Backwards
   • Individual-based data
     • Descriptive
       • Case series
         • —Case Series report new diseases or health related problems. —They may provide some descriptive data on exposures to potential causal factors —A group or series of case reports involving patients who were given similar treatment. —Usually contain detailed information about the individual patients.
     • Analytic
       • CS
         • —Measure existing disease and current exposure levels. —Sample without knowledge of Exposure or Disease —Sample at one point in time —Mostly prevalence studies/surveys —Methods include: —Face-to-face interviews —self-administered questionnaires —Physical examinations —Telephone interviews
         • Major use
           • —Exploratory studies —When little is known about the potential predictors —Frequently used at preliminary or planning stage of investigation — —Basic steps —Formulate one or more hypotheses —Determine the target population —select a sample —Determine appropriate measures of exposure and outcome —Collect and analyze data
         • Advantages
           • —Hypothesis generation —Estimate prevalence and sometimes rates —Estimate exposure proportions in the population —Can study multiple exposures or multiple outcomes or diseases —Relatively easy, quick and inexpensive —No issue of subjecting any animals or producers to particular treatments —Often good first step for new study issue
         • Disadvantages
           • —Impractical for rare diseases —Measures prevalence, not incidence —Not a useful type of study for establishing causal relationships —Confounding is difficult to control —Problems with temporal sequence of data —hard to determine the onset of disease —miss diseases still in latent period —recall of previous exposure may be faulty
         • Steps
           • —Formulate one or more hypotheses —Determine the target population —select a sample —Determine appropriate measures of exposure and outcome —Collect and analyze data
       • Case control studies
         • —Case-Control Studies identify existing disease/s and look back in previous years to identify previous exposures to causal factors. —Cases are those who have a disease. —Controls are those without a disease. —Analyses examine if exposure levels are different between the groups. —Methods   —Questioning  —Reviewing records  —Biological samples —Interviewing relatives or others
         • Advantages
           • —Best design for rare diseases —Can be accomplished quickly since events of interest have already occurred —Can study several potential exposures at the same time —Lends itself well to hospital-based studies and outbreaks
         • Disadvantages
           • —Problems with temporal sequence of data — —Hard to determine the onset of disease Miss diseases still in latent period — —Can’t calculate incidence, population relative risk or attributable risk — —HIGH potential for bias —Selection bias —Recall bias p
         • Steps
           • —What are the hypotheses? - Define the question — Who are the cases? could be histological / microbiological / clinical pattern / coding —specific criteria: focussed —Define source of cases —Who are the controls? —Source, matching, number of control groups, number of controls per case — Source is dependant on source of cases —Estimating exposure —Analysis —Interpretation
       • Cohort studies
         • —Subjects with an exposure to a causal factor are identified —Subjects are disease free at the beginning —Incidence of a disease over time is compared with that of controls (persons who do not have the exposure). —In a longitudinal study, subjects are followed over time with continuous or repeated monitoring of risk factors or health outcomes, or both.
         • Advantages
           • —Assesses exposure and can deal with changes in exposure —May be only design if exposure needs to be measured directly —Allows study of rare exposures —Allows study of  causation / temporal sequence —Only way to get prospective information for rapidly fatal diseases —Good for establishing temporal sequence and natural history of disease —Can examine multiple outcomes linked to exposure - often find other effects than —Can estimate overall and specific disease rates —Lower potential for bias than a case-control study - no recall and selection bias —Results are considered more conclusive than results from case-control studies —The longer a cohort study continues, the stronger it can become —Impractical for rare diseases —may not be statistically significant —Larger sample size than case-control —Data collection is usually very expensive —Long time commitment for follow-up
         • Disadvantages
           • —Impractical for rare diseases —may not be statistically significant —Larger sample size than case-control —Data collection is usually very expensive —Long time commitment for follow-up

2. Experimental

3. Metaanalysis

   • —Pools results across multiple studies —A review article with quantitative summary —Typically combines results of several experimental studies —Useful for combining small studies —Studies should have same or similar treatments —Pools results to get single measure of effect —Beware: meta-analyses combining experimental and observational designs —Dependent upon articles reporting sufficient data (N, effect measure, variance)

4. Which study to choose?

   • Øwhat is  the research question/ objective ØTime available for study ØResources available for the study ØCommon/rare disease or production problem ØType of outcome of interest ØQuality of data from various sources ØOften there are multiple approaches which will all work ØChoosing an established design gives you a huge head start in design, analysis and eliminating biases

5. Choosing design?

   • pWhat is the objective of the study p p Resources available for the study p p Is it a common/rare disease p p Type of outcome of interest p  Quality of data from various sources