Critically Appraised Topic Flashcards
What is selection bias?
= error is assigning indidivuals to groups leading to differences that may influence the outcome
The subjects are not representative of the population
Selection bias: what is sampling bias?
= selected subjects are not representative of the population
Selection bias: what is volunteer bias?
= the volunteer subjects are not representative of the population
Selection bias: what is non-responder bias?
= the respondants (subjects) are not representative of the population
Selection bias may be a particular problem in which type of observation study?
= cohort studies
What is a cohort study?
= involves finding a sample that do not have the outcome (e.g., lung cancer)
Group them according to exposure
Follow them forward to see who develops the outcome
What is a case control study?
= find a sample that HAS the outcome AND, find a control sample that doesn’t have the outcome
Look to the past to find out who was exposed to the exposure
What is recall bias?
= difference in the accuracy of recollection of study participants, possible due to whether they have the outcome of not
(e.g., patient with mesothelioma may try harder to remember exposure to asbestos than a control patient)
In which type of observational study is recall bias a particular problem?
= case-control study
What is publication bias?
= failure to publish/ include results from valid studies, often because they show a negative or uninteresting result
In which type of studies may publication bias be a problem? (2)
- meta-analysis
- systematic reviews
(studies showing negative results may be excluded)
What is work-up bias (verification bias)?
= this refers to the gold-standard test being preformed more frequently in patients who have already had a positive result from the new test
e.g.,those with a +ve d-dimer may be assessed further using ultrasonography, whereas those with a -ve d-dimer will be assessed with 3-month follow up. Patients who had a DVT, but neg. d-dimer may not have been diagnosed by 3-month follow-up. This risks underestimating the no of false negatives, and thus may overestimate the sensitivity of a new test
What is an expectation bias (pygmalion effect)?
And how is it tackled?
= observers may subconsciously measure or report data in a way that favours the expected study outcome
= avoided by blinding
What the hawthorne effect?
= group changing it’s behaviour due to knowledge that it is being studied
What is procedure bias?
= subjects in differed groups receive different treatment, other than just interventions
(e.g., elderly patients could receive human contact along with intervention, whereas control patients get no extra human contact)
What is length-time bias?
= screening over-represents less-aggressive disease
e.g., aggressive tumours have a shorted asymptomatic (screening) period, so screening is less likely to detect aggressive tumours
What is lead-time bias?
= early diagnosis appears to prolong survival
What is late-look bias?
= gathering information at an inappropriate time
e.g., studying a fatal disease many years later when some patients may have died
Advantages of a cohort study (3)
- best information about causation of disease, as can work out incidence
- able to examine range of outcomes from a particular exposure
- good for rare exposures - can select those exposed to certain things
Disadvantages of a cohort study? (8)
- long follow-up period, expensive & time consuming
- bad for rare outcomes - would require enormous sample size
- bad for long latent periods (have to wait ffor disease to develop)
- misclassified exposure (lying)
- different follow up for exposed/ non-exposed
- outcome assessors not blind to exposre category
- selection bias - subjects not representative of the population as a whole
- confounders not recognised/ addressed
Advantages of case control studies? (3)
- simple/ easy to consuct, does not require long follow up
- good for rare outcomes - can select patients with certain disease
- good for long latent periods - finding patients after disease developed, not waiting after exposure
Disadvantages of a case control study? (5)
- bad for rare exposures - would require an enormous sample size
- controls may not represent the population where the sample is from
- cases don’t represent full disease spectrum (e.g., fatal vs. non-fatal lung cancers)
- recall bias - different reporting of exposure drom different groups
- confounders not recognised/ addressed
What is equipoise?
= the situation where the researchers have no preference between the 2 interventions being studied in the trial
Why is equipoise important?
= important as it is unethical to run a trial when you alread known one intervention is inferior, becasue you are exposing those patients to sub-optimal treatment, and perhaps unnecessary risk of harm
