• Understanding Research: Bias

    by Cian O'Brien. Last modified: 13/04/14


    What is Bias?

    • Bias is a systematic error in epidemiological study that results in an incorrect estimate of the association between exposure and risk of disease
    • Results from systematic errors in research methodology
    • Limited scope exists for the adjustment of most forms of bias at the analysis stage
    • Careful consideration and control of the ways in which bias may be introduced during the design and conduct of the study is essential in order to limit the effects on the validity of the study results

    Types of Bias

    Information Bias

    • Results from systematic differences in the way data in exposure or outcome are obtained from various study groups

    Observer Bias

    • When there are systematic difference in the way information is collected for the groups being studied
    • Observer bias may occur as a result of the investigator’s prior knowledge of the hypothesis under investigation or knowledge of an individual’s exposure or disease status
    • Such information may result in differences in the way information is collected, measured or interpretation by the investigator for each of the study groups

    Minimizing observer bias

    • Where possible, observers should be blinded to the exposure and disease status of the individual
    • Blind observers to the hypothesis under investigation
    • In a RCT: blind investigators and participants to treatment and control group (double blind RCT)
    • Development of a protocol for the collection, measurement and interpretation of information
    • Use of standardized questionnaires
    • Training of interviewers

    Losses to follow-up

    • Particular problem with cohort studies
    • Bias may be introduced if the individuals lost-to-follow up differ with respect to the exposure and outcome from those persons who remain in the study

    Recall Bias

    • In a case-control study, data on exposure are collected retrospectively.
    • Quality of data is determined to a large extent by the patient’s ability to accurately recall past exposures
    • Recall bias may occur when the information provided on exposure is different between the cases and controls
    • Recall bias may result in either under/overestimate of the association between exposure and outcome
    • Methods to minimize recall bias:
      • The collection of exposure data from work/medical records or to blind the study participants as to the hypothesis under investigation

    Selection Bias

    • Occurs when the 2 groups being compared differ systematically
    • There are difference in the characteristics between those who are selected for a study and those who are not selected, and those characteristics are related to either the exposure or outcome under investigation
    • Case-control Studies
      • Highly vulnerable to selection bias, particularly in the control group
      • The purpose of the control group is to estimate exposure in the base population
      • Selection bias results if control selection is not neutral with respect to exposure

    Validity – absence of bias
    Internal Validity – whether the study provides an unbiased estimate of what it claims to estimate
    Example: suppose investigators say that they are studying Alzheimer’s disease in North Carolina elderly.  If the study findings do in fact accurately depict the situation among NC elderly, then the study is (internally) valid.
    External Validity – whether the results from the study can be generalized to some other population
    Example: do the results from a study carried out in North Carolina apply to people in the southeast U.S. as a whole, the entire U.S., all of North America, all industrialized countries, etc.? 
    Positive Bias – observed value is higher than the true value
    Negative Bias – observed value is lower than the true value
    Bias towards the null – observed value is closer to 1.0 than is the true value
    Bias away from the null – observed value is farther from 1.0 than is the true value

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    Cian O'Brien

    Cian O'Brien

    Cian is an Irish trained Emergency Medical Technician, Registered General Nurse and holds a Masters degree in Public Health from University College Cork, Ireland. His research interests include prehospital care and marathon medicine.

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