Why evidence based medicine Flashcards
why does EBM matter to clinicians
Revalidation Medical knowledge Practice-based learning and improvement Interpersonal and communication skills Professionalism
what are possible explanations for observed associations
chance, bias, confounding, causation
what is chance (coincidence)
Make inference from samples rather than whole populations , therefore we are estimating. Sample size. Power calculations. Probability (P) values and statistical significance (p=1/20. The role of chance can be assessed by appropriate statistical significance tests or a confidence interval (rage of values in which true value lies)
Use of EBM
Clinical findings. Aetiology- causes of disease. Clinical manifestations of disease. Differential diagnosis- and selecting which are most likely. Diagnostic tests. Prognosis- likely clinical course of events after diagnosis. Therapy- select most effective treatments. Prevention- identifying and modifying risk factors, and how to diagnose disease early by screening
what is the heirachy of studies
Systematic reviews and meta-analysis. Randomised controlled trials. Cohort studies. Case-control studies. Ecological studies. Descriptive/cross-sectional studies. Case report/series
define association
refers to the statistical dependence between two variables. Consider chance, bias, confounding, cause (CBCC)
what is bias
Systematic error. Consequence of defects in the design or execution of the study. Selection bias e.g. Researcher choosing preferable patients, loss of follow up, non-response
Measurement bias e.g. Faulty equipment, recall. Observer bias – Prevented by Double Blinding. Responder bias – Prevented by Single Blinding
define confounding
mixing of effects between exposure, the disease and a third factor.Account for confounding using matching, randomisation, stratification and multivariate analysis
what is a potential confounder
any factor which is believed to have a real effect on the risk of the disease under investigation and is also related to the risk factor under investigation
what are common confounders
age, sex, socio-economic status and geography
what is causal-effect
judgement of a cause-effect relationship based on a chain of logic that addresses two main areas: Observed association between an exposure and a disease is valid. Totality of evidence taken from a number of sources supports a judgement of causality. Must eliminate chance, bias and confounding before considering causal effect
what are cohort studies
carried out over a period of time - prospective
what are case control studies
compares cases to controls. retrospective. information on past exposure to possible risk factors is obtained for cases and controls, and exposure in the cases is compared with that in the controls.
what is a case report
follow up of individual patient
what is an Ecological study
observationalstudydefinedby the level at which data are analysed, namely at the population or group level, rather than individual level. Ecological studiesare often used to measure prevalence and incidence of disease, particularly when disease is rare.
what are descriptive/cross sectional studies
Observationalstudythat analyses data collected from a population, or a representative subset, at a specific point in time
list the Bradford-Hill criteria for establishing causation
strength of association. consistency. specificity. temporal relationship. dose-response relationship. plausibility. coherence. experimental evidence. analogy
what is strength (Bradford hill criteria)
Measured by the magnitude of relative risk. Strong association is more likely to be causal and a weak association could be more easily the result of confounding or bias(but doesn’t rule out a causal relationship). E.g passive smoking and lung cancer
what is consistency(Bradford hill criteria)
Similar results found in different studies since it is unlikely that they were all subject to the same type of errors. However, lack of consistency does not exclude a causal association since different exposure levels, different populations using different designs may reduce the impact of the causal factor
what is specificity (Bradford hill criteria)
A particular exposure increasing the risk of a certain disease but not the risk of other diseases shows strong evidence in favour of a cause-effect relationship e.g. Mesothelioma. However, one-to-one relationships between exposure and disease are rare. Lack of specificity should not be used to refute a causal relationship; e.g cigarette smoking causes many diseases.
what is temporal relationship (Bradford hill criteria)
Essential criterion. For a putative risk factor to be the cause of a disease, it has to precede the disease. Generally easier to establish from cohort studies but rather difficult from case-control or cross-sectional studies when measurements of the cause and the effect are made at the same time.
what is dose response relationship (Bradford hill criteria)
Increasing levels of exposure lead to increasing risks of disease shows evidence of a causal effect. Some however show a jump rather than a monotonic trend.
what is plausibility(Bradford hill criteria)
Association is more likely if consistent with other knowledge e.g animal experiments, biological mechanisms. Should not be taken too seriously because lack of plausibility may just reflect lack of scientific knowledge.
what is coherence (Bradford hill criteria)
Cause and effect interpretation does not conflict with what is known of the natural history. However, absence of coherent information (e.g. conflicting information) should not be taken as evidence against an association being causal.
what is experimental evidence (Bradford hill criteria)
On humans (rarely available) or animals
what is analogy (Bradford hill criteria)
Provides a source of more elaborate hypotheses about the association in question. Absence only reflects lack of imagination or experience.
