Bias

Question 1

Error in assigning inidividuals to groups leading to differences which may influence outcome.

Answer 1

Selection bias

Question 2

Name some subtypes of selection bias

Answer 2

• Sampling bias
• Volunteer bias
• Non-responder bias
• Loss to follow up bias
• Prevance/incidence bias (Neyman) bias.
• Admission Bias (Berkson’s Bias)
• Healthy worker Effect

Question 3

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.

Answer 3

Recall bias

Question 4

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.

Answer 4

Publication bias

Question 5

In studies which compare new diagnostic tests with gold standard tests, work-up bias can be an issue. 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. Sometimes work-up bias cannot be avoided, in these cases it must be adjusted for by the researchers.

Answer 5

Work-up bias (verification bias)

Question 6

Only a problem in non-blinded trials. Observers may subconsciously measure or report data in a way that favours the expected study outcome.

Answer 6

Expectation Bias

(A.K.A. Pygmalion effect)

Question 7

Describes a group changing it’s behaviour due to the knowledge that it is being studied

Answer 7

Hawthorne effect

Question 8

Gathering information at an inappropriate time e.g. studying a fatal disease many years later when some of the patients may have died already

Answer 8

Late-look bias

Question 9

Occurs when subjects in different groups receive different treatment

Answer 9

Procedure bias

Question 10

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.

Answer 10

Lead-time bias

Question 11

A form of bias that occurs when measurement of information differs among study groups.

Examples include:

• Recall bias, reporting bias
• Diagnostic bias
• Hawthorne effect
• Errors in measurement

Answer 11

Information bias

Question 12

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

Answer 12

Publication bias

Question 13

Distortion of exposure, disease relation by some other factor

Answer 13

Confounding bias

Question 14

The subjects are not representative of the population.

Answer 14

Sampling bias

Question 15

When a study is investigating a condition that is characterised by early fatalities or silent cases. It results from missed cases being omitted from calculations

Answer 15

Prevalance/incidence Bias

(a.k.a. Neyman bias)

Question 16

Cases and controls in a hospital case control study are systematically different from one another because the combination of exposure to risk and occurrence of disease increases the likelihood of being admitted to the hospital

Answer 16

Admission bias

(Berkson’s bias)