Bias Flashcards
Define bias
the situation in a trial where one outcome is systematically favoured.
What is selection bias?
Error in assigning individuals to groups leading to differences which may influence the outcome.
A subtype of selection bias is sampling bias. What is this?
The subjects are not representative of the population.
What may sampling bias be due to?
Volunteer bias or non-responder bias.
What is volunteer bias?
E.g. A study looking at the prevalence of Chlamydia in the student population. Students who are at risk of Chlamydia may be more, or less, likely to participate in the study.
What is non-responder bias?
If a survey on dietary habits was sent out in the post to random households it is likely that the people who didn’t respond would have poorer diets than those who did.
What is recall bias?
Difference in the accuracy of the recollections retrieved by study participants, possibly due to whether they have disorder or not.
E.g. a patient with lung cancer may search their memories more thoroughly for a history of asbestos exposure than someone in the control group. A particular problem in case-control studies.
What is publication bias?
Failure to publish results from valid studies, often as they showed a negative or uninteresting result.
Important in meta-analyses where studies showing negative results may be excluded.
What is work-up bias (verification bias)?
Sometimes clinicians may be reluctant to order the gold standard test unless the new test is positive, as the gold standard test may be invasive (e.g. tissue biopsy).
This approach can seriously distort the results of a study, and alter values such as specificity and sensitivity.
In what type of studies may work-up bias be a problem?
In studies which compare new diagnostic tests with gold standard tests.
What is expectation bias?
Observers may subconsciously measure or report data in a way that favours the expected study outcome.
When is expectation bias only a problem?
In non-blinded trials.
What is the Hawthorne effect?
Describes a group changing it’s behaviour due to the knowledge that it is being studied
What is late-look bias?
Gathering information at an inappropriate time e.g. studying a fatal disease many years later when some of the patients may have died already.
What is procedure bias?
Occurs when subjects in different groups receive different treatment.
