Timing of bias
Type of bias
Explanation
Before a clinical trial begins
Selection bias
Study subjects improperly chosen
Channeling bias
Prognosis of study subject influences group or cohort assignment
During a clinical trial
Interviewer bias
Variability in manner by which information is gathered or recorded from subject to subject
Chronology bias
Comparison group includes historical controls
Performance bias
Exposure to other factors apart from intervention
Recall bias
Variability in accuracy of subject retrieval of prior experience
Attrition (Transfer) bias
Excess study subjects lost to follow-up
After a clinical trial
Detection bias
Partiality when assessing outcomes
Expectation bias
Expecting a certain result can influence assessment of outcomes
Correlation bias
Correlation improperly implied as causation
Publication (Citation) bias
Favor distribution of studies showing a difference between groups as opposed to studies supporting null hypothesis
Table 1.2
Bias in observational studies
Type of bias | Explanation |
|---|---|
Self-selection bias | Social, cultural, linguistic, and health values or barriers may promote or hinder patient enrollment in a study, screening program, or measured point of care |
Lead-time bias | Influence of a screening program on calculation of stage-specific survival rates: patients living in areas with a screening program for disease X may be diagnosed earlier and therefore concluded to live longer with disease X than age-matched counterparts living in an area without a screening program |
Ecological fallacy | Assumption that observed associations can be extrapolated from a population to an individual |
Exclusion bias | Evaluating data leaving a particular group or groups out of a sample population |
Referral bias | Influence of referral from primary to tertiary and subspecialty centers on makeup of a study population: an observational study consisting of a patient population from a tertiary or subspecialty clinic may have higher percentage of complex or severe cases and adverse outcomes |
Spectrum bias | Change in the performance characteristics of a particular test when applied to different patient subgroups |
Levels of Evidence and Grading of Recommendations
When determining what is “best evidence,” the most rigorous study designs are, by nature, those least prone to errors from chance, confounding, and bias. Table 1.3 illustrates how study design directly contributes to a study’s position within the hierarchy of evidence. It should be noted that this tier system for assigning proportionate value to study design is by no means absolute; a flawed randomized control trial, for example (e.g., with inherent bias, poor design or control of external variables), would certainly not hold the same weight as a rigorously designed and executed study. Rather, this is to serve as a rough outline for how practitioners should approach impact of study design, and how committees who review available data establish guidelines and clinical recommendations .
Table 1.3
Levels of evidence
OCEBM level | Therapy/prevention, etiology/harm | Prognosis | Diagnosis | Differential diagnosis/symptom prevalence study | Economic and decision analyses |
|---|---|---|---|---|---|
1a | SR (with homogeneity) of RCTs | SR (with homogeneity) of inception cohort studies; CDR″ validated in different populations | SR (with homogeneity) of level 1 diagnostic studies; CDR″ with 1b studies from different clinical centers | SR (with homogeneity) of prospective cohort studies | SR (with homogeneity) of level 1 economic studies |
1b | Individual RCT (with narrow confidence interval) | Individual inception cohort study with >80 % follow-up; CDR″ validated in a single population | Validating cohort study with good reference standards; or CDR″ tested within one clinical center | Prospective cohort study with good follow-up | Analysis based on clinically sensible costs or alternatives; systematic review(s) of the evidence; and including multi-way sensitivity analyses |
1c | All or none | All or none case series | Absolute SpPins and SnNouts | All or none case series | Absolute better-value or worse-value analyses |
2a | SR (with homogeneity) of cohort studies | SR (with homogeneity) of either retrospective cohort studies or untreated control groups in RCTs | SR (with homogeneity) of level >2 diagnostic studies | SR (with homogeneity) of 2b and better studies | SR (with homogeneity) of level >2 economic studies |
2b | Individual cohort study (including low-quality RCT; e.g., <80 % follow-up)
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