the science of EBM Flashcards
what are the different types of observational study?
- case control
- cohort
- cross sectional
what are examples of experimental studies?
- randomised and non randomised trials
what are the principle of evidence based medicine?
1) patient dilema
2) ask
3) acquire
4) appraise
5) apply
6) act and asses.
what are examples of reviews?
- expert opinion
- systemic review
- meta-analysis
what are the 8 levels of evidence?
what are 5 factors that might decrease the quality of evidence?
- study limitations
- inconsistency of results
- indirectness of evidence
- imprecision
- publication bias
what are 3 factors that might increase the quality of evidence?
- large magnitude of effect
- plausible confounding, which would reduce a demonstrated effect
- dose response gradient
what are the advantages of case studies?
- quick and cheap to conduct
- rapid publication
- early indications of problems
- can help detect new drug side effects and potential uses
what are the 4 disadvantages of case studies?
- statistically weak
- no control group
- vert small numbers of patients
- cases may not be generalised to the wider population
wha are the advantages of cross sectional survey?
- cheap and simple
- ethically safe
- useful for planning purposes
what are the disadvantages of cross sectional study?
- cause and effect?
- volunteer bias
- unequal distribution of confounders
what are 2 complications that may arise in studies?
confounders = uncontrolled extraneous variables
- observation, smokers tend to have smaller babies than non smokers
SPURIOUS ASSOCIATION
- it understand harmful to the foetus?
= - initial studies showed that babies exposed to ultrasound has a lower birth weight
- later studies failed to confirm this observation.
how do case control studies work?
advantages of case control studies?
- youcan look at multiple rosk factors at once.
- good for studying rare conditions or diseases with long latent period
- useful as initial studies to establish an association
- do not require a long follow up period
disadvantages of case control study?
- retrospective study which relies on patient recall to determine exposure or patient records
- confounders
- selection of control group is difficult
- unlikely to detect rare causes of disease
how does a cohort study work?
what are the advantages of cohort study?
- ethically safe
- subjects can be matched
- can show cause precess the effect
- easier and cheaper than a randomised controlled trial
what are the disadvantages of cohort study?
- high drop out rate
- exposure may be linked to hidden confounder
- blinding is difficult
- outcome of interest may take a long time to occur
how does a randomised controlled trial work?
how does a double blind method work?
design modifiers:
single blind?
double blind?
cross over?
placebo controlled?
design features of clinical research:
parallel group comparison?
paired (matched) comparison?
within subject comparison?
advantages of randomised controlled trial?
- unbiased distribution of confounders
- clearly identified populations
- randomisation helps statistical analysis
- more likely to be blinded
disadvantages of randomised controlled trial?
- expensive
- volunteer bias
- ethical issues if treatment group are seen to respond badly or better than expected
what are the 5 problems associated with RCT’s?
- impossible = with treatments for very rare disease where the number of patients is too limited
- unnecessary = when a treatment produces a ‘dramatic’ benefit - imatinib for chronic myeloid leukaemia
- stopping trials early= interim analysis of trials is now commonly undertaken to assess whether the treatment is showing benefit if the trial can be stopped early
- resources = the costs of RCT’s are substantial in money, time and energy
- generalisability = RCT’s are often carried out on specific types of patients for a relativey short period of time.
how do you establish cause and effect?
1) is the association due to a chance occurrence?
2) is it due to a flaw in the methodology?
3) is it due to another factor which is linked to both the exposure and the outcome
what is the Bradford Hill criterion linking smoking to lung cancer?
what are the advantages and disadvantages of expert (narrative) review?
advantages = comprehensive survey , answers a specific question
disadvantages = expert bias
what is a systematic review?
a systematic review attempts to identify, appraise and synthesise all the empirical evidence that meets pre- specified eligibility criteria to answer a given research question.
- it is a comprehensive literature search that identifies all similar and relevant studies that satisfy pre-defined:
- inclusion criteria
- exclusion criteria
how do researchers conduct a systematic review to minimise bias?
researchers conducting systemic reviews use explicit methods aimed at minimising bias, in order coproduce more reliable findings that can be used to inform decision making.
describe the vitamin C for preventing and treating the common cold study?
how do you practise EBM?
- craft a clinical question
- search the medical literature
- find the study that will best answer the question
- perform a critical appraisal
- determine how the results will help you care for your patient
- evaluate the results in your patient or population
how can you identify different trials?
why are systematic review views as the gold standard?
because the avoid/minimise bias
what are the advantages of systematic review?
- uses explicit and reproducible methodology
- many different studies can be compared and contrasted directly with each other to determine the overall findings
- reduces bias as it takes into account the range of views and findings
- less costly to review previous studies than to initiate a new study
what are the disadvantages of systematic review?
- dependant on screening strategy
- can be hard to compare results of studies that have completed in very different ways
- publication bias
- very time consuming
what is met analysis?
combine qualitative and quantitive study data from several selected studies to develop a single conclusion that has a greater statistical power.
4 stages of meta analysis
1) establish statistical significance with studies that have conflicting results
2) develop a more accurate estimate of effect magnitude
3) provide a more complex analysis of harms , safety data and benefits
4) examine subgroups with individual numbers that are not statistically significant
advantages of meta analysis
- greater statistical power
- greater ability to extrapolate to the general population
- statistical analysis is more objective than narrative analysis
- more effect to present a combined result.
disadvantages of met analysis
- individual studies do not necessarily all use the same methodology
- heterogeneity of study populations
- requires advanced statistical techniques
- very time consuming
