Interpreting epidemiological findings (1 of 3) Flashcards
Koch’s postulates for if a pathogen is causative of a disease? (old)
- microorganisms must be found in abundance in all organisms with the disease, but not in healthy organisms
- microorganisms must be isolated from a diseased organism and grown in pure culture
- the cultured microorganism should caused disease when introduced into a healthy organism
- microorganism must be reisolated from the inoculated, diseased experimental host and identified as being identical to the original specific
what is the problematic postulate from koch?
- the cultured microorganism should caused disease when introduced into a healthy organism
- unethical to intentionally subject someone to an infection
bradford-hill causative criteria (9)
- strength
- consistency
- specificity
- temporality
- biological gradient
- plausibility
- coherence
- experiment
- analogy
strength aspect of bradford-hill causative criteria
when there is a stronger relationship between and exposure and outcome, it makes it more likely there’s some causative element going on
consistency aspect of bradford-hill causative criteria
- consistent findings across setting tended to rule out error or fallacies that might befall one/two studies
- use carefully and with difficulty to determine causation
specificity aspect of bradford-hill causative criteria
- most criticised of BH’s points
- he does assert that a specific disease arising among specific workers is valuable if supporting the argument for causality
- hard to apply when issue is multifactorial
temporality aspect of bradford-hill causative criteria
it’s insufficient for exposure A and outcome B to coexist
- A must commonly precede B
- cross-sectional studies can’t always assess which came first
- longitudinal studies are useful here
biological gradient aspect of bradford-hill causative criteria
idea of dosage
- e.g. smoking more causes more lung cancer
plausibility aspect of bradford-hill causative criteria
- relationship shouldn’t be implausible
- where deficient understanding this may be hard to assess
coherency aspect of bradford-hill causative criteria
association ought to be consistent with existing theory and knowledge
- overlaps to some extent with biological plausibility
experiment aspect of bradford-hill causative criteria
- evidence from experimentation should be supportive of the proposed link
- though scientifically desirable, experimentation is often not ethical when dealing with public health issues
analogy aspect of bradford-hill causative criteria
- drawing upon analogous findings, we may make inference on the relationship
- relies on prior knowledge that doesn’t always exist
external validity means
findings from a study can be applied to other settings, not where the research was conducted
bias
an inference is valid when there is no bias
- bias is any trend in the collection, analysis, interpretation, publication or review of data that can lead to conclusions that are systematically different from the truth
which errors are allowable
random
- large sample cancels out errors
- systematic errors are the problem
three types of bias
- selection bias
- information bias
- confounding
External validity is synonymous with:
generalisability
definition of internal validity.
The extent to which findings accurately describe the relationship between exposure and outcome in the context of the study
A study, which is designed very well, and implemented equally well - but which is carried out a single teaching hospital in London can be said to have high:
Internal validity
The findings of a study which is designed well but which loses more participants than expected are submitted for peer review. What might be called into question by the peer reviewers?
internal validity
The findings of a study which is designed poorly but implemented exceptionally well, is submitted for peer review. What might be called into question by the peer reviewers?
internal validity
An international multicentre randomised controlled trial with triple blinding is reported in the literature. On the basis of this sentence alone, what is likely?
Good external validity
selection bias occurs when:
individuals or groups in a study differ systematically from the population of interest leading to a systematic error in an association or outcome.
- e.g. study on consumption of red meet causing cancer only advertised in vegetarian shops might impact cohort composition
example of selection bias
Berkson’s bias
- hospital- based control study using hospitals patients
- healthy worker effect - active workers more likely to be healthy compared to those who have retired/ stopped working
ways to reduced selection bias
- controls representative of target population
- minimise non-response
- compare respondent with on-respondents
information bias
- misclassification of exposure or disease status or both
- any systematic difference from the truth that arises in the collection, recall, recording and handling of information in a study, including how missing data is dealt with
interviewer bias is an issue for which type of bias?
information bias
- mitigated if interviewer doesn’t know the health status of participant
recall bias is prevented how
- using non-subjective methods for assessment e.g. biomarkers and medical records
non-differential misclassification does what to odds ratio?
- always biased towards the null
response bias describes what?
the tendency of participants to respond to questions in a way that is more socially acceptable.
–> This not a type of selection bias.
information bias can include
- recall bias
- response bias
- interviewer bias
- diagnostic bias
Where cases and controls equally mis-report their exposures, this would more likely be described as:
Non differential misclassification
Where cases and controls unequally mis-report their exposures AND in such a way that the overall consequence is an association that tends away from the null, this is best described as:
Information bias
what is confounding?
the effect of an extraneous variable or wholly or partially accounts for the apparent effect of the study exposure, or that masks an underlying true association
e.g. birth order being associated with a baby having down syndrome is actually masking the increasing age of mothers when they give birth
4 ways to identify confounders
- Knowledge of the subject matter: you could undertake an evidence review and see what other people have proposed or found.
- Test by the three criteria: associated with exposure in the source population; associated with the outcome in the absence of the exposure; not a consequence of the exposure.
- Stratification: we can look at the difference of apparent effect within different population strata.
- Compare the crude and adjust statistical estimates.
what is effect modification?
when the strength of the association varies over different levels of a third variable
should you control effect modification/
no - like confounding it’s naturally occurring
- way results reported should take it into account
tests for effect modification
- Breslow-Day test
- Q test
- Interaction terms in regression models
what are the two critical points for why differentiation between cofounding and effect modification are important?
- Addressing a confounded relationship by addressing the exposure exclusively is very unlikely to yield a gain. In example (3), stopping a mother from smoking is unlikely to reverse the childhood mental health outcome.
- Addressing an exposure where effect modification is apparent may be useful: it may be possible to target an intervention into a more homogeneous pool of participants where a greater impact will be yielded
what is a highly effective way for us to identify possible confounding.
Multivariate regression
