Critical Thinking - Lecture Twenty-Five Flashcards
Selection Bias
Bias
Bias is any systematic error in an epidemiological study that results in an incorrect estimate of the association between exposure and risk of disease.
Findings of systematic error
Over-estimation, under-estimation or not affected
When is systematic error important?
When critically appraising scientific literature, in evidence-based practice, when considering studies reported in the media and undertaking research
What can be done about bias?
Selection and information bias can only be controlled during the design and data collection phases of a study
What must investigators do during planning stages of a study?
Identify the potential sources of bias in the proposed study and the possible ways to minimise these potential biases
Selection bias
Selection bias occurs when there is a systematic difference between the people who are included in a study and those who are not, or when study and comparison groups are selected inappropriately or using different criteria
Three important questions when planning stages in a study
How do we recruit people into epidemiological studies?
Does everyone agree to participate?
Does everyone remain in the study?
Volunteering
People who volunteer might be different to people who don’t volunteer and cause bias
Representative sample
Randomly select people from a defined population that they’re interested in
Selection bias and different study designs
While all analytic studies are subject to bias, each design has particular types to which it is inherently most vulnerable
Cross-sectional studies
Exposures and/or outcomes assessed at one point in time, and measuring prevalence
Considering selection bias
Who entered the study?, is the sample representative of the source population?, what is the response rate?
If the sample was not representative of the source population is the prevalence likely to be
Over-estimated?
Under-estimated?
Not affected?
Selection bias: case-control studies
Selection bias occurs when there is a systematic difference between the people who are included in a study and those who are not, or when study and comparison groups are selected inappropriately or using different criteria
How are participants selected in the case-control studies
On the basis of their outcome status
How can bias occur in terms of selection for case-control studies?
If the selection is dependant on their exposure status
How to avoid potential biases in selection of cases
Ensure high participation
Clearly defined population of interest
Reliable way of ascertaining all cases or a representative sample of cases
Potential biases in selection of controls
When controls are not representative of the population which gave rise to cases (in place or time), if inclusion/exclusion criteria differ between cases and controls and if non-participation of eligible controls
Controls not representative of the population which gave rise to cases (in place or time)
Ensure controls are from the same defined population as the cases over the same time period
If inclusion/exclusion criteria differ between cases and controls
Same inclusion and exclusion criteria for cases and controls
If non-participation of eligible controls
Ensure high participation
Selection bias in loss of follow up
Study goes over time so people may not remain in the study
If loss to follow-up is related to both the exposure and the outcome this can lead to bias
How can the selection of comparison and exposed groups cause bias?
When they are selected separately
