Bias Flashcards
Error in assigning individuals to groups leading to differences which may influence the outcome
Selection bias
subjects in a sample are not representative of the population
Sampling bias
Give an example of volunteer bias
Example: 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-resopnder bias? give an example?
Example: 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.
Difference in the accuracy of the recollections retrieved by study participants, possibly due to whether they have disorder or not
Recall 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.
Publication bias
Describe “work-up bias” or “verification bias”
Studies which compare new diagnostic tests with gold standard tests
=> clinicians 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 can distort the results of a study, and alter values such as specificity and sensitivity.
A problem in non-blinded trials.
Observers may subconsciously measure or report data in a way that favours the expected study outcome.
Expectation bias (Pygmalion effect)
Describes a group changing it’s behaviour due to the knowledge that it is being studied
Hawthorne effect
Gathering information at an inappropriate time e.g. studying a fatal disease many years later when some of the patients may have died already
Late look bias
Occurs when subjects in different groups receive different treatment
Procedure bias
Occurs when two tests for a disease are compared, the new test diagnoses the disease earlier, but there is no effect on the outcome of the disease
Lead time bias
