Selection Bias

Question 1

what is bais and why does it matter?

Answer 1

bias is any systematic error that results in an incorrect estimate of the association between exposure and risk of outcome
- it has a pattern to it (consistent), eg. using broken machinery to measure a value so its always ten above the actual value

matters because is results in an incorrect estimate

Question 2

when is systematic error important and how much are the findings influenced?

Answer 2

Finding could be overestimate, underestimated or not affected. we want to know in which direction and by how much the findings are incorrect.

Important when:
- critically appraising scientific literature
- in evidence-based practice
- considering studies reported in the media
- undertaking research

Question 3

what can we do about bias?

Answer 3

• selection and information bias can only be controlled during the design and data collection phases of the study, once it is in the results, it there’s no way to get rid of it
• so at the planning stages investigators must:
  
  • identify the potential sources of bias in the proposed study
    - identify possible ways to minimise these potential biases

Question 4

what are the three potential sources of bias?

Answer 4

• selection bias
• information bias
• publication bias

Question 5

when does systematic bias occur and at what stages do we have to consider it?

Answer 5

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

1. How do we recruit people into epidemiological studies?
   - volunteers (if there is a reason they are volunteering and its related to what you are investigating, this could introduce bias)
   - randomly selecting people from a defined population (much better way to select people, but depending on how its carried out there still could be bias)
2. Does everyone agree to participate?
   - if not, is it related to what you are studying? could introduce bias
3. Does everyone remain in the study?
   - if not, why? does it relate to your study and will it introduce bias?

Question 6

how do you minimise loss to follow-up?

Answer 6

• alternative contact details obtained at the start of the study
• maintaining regular contact
• making several attempts to contact

Question 7

describe how selection bias works in cross-sectional studies

Answer 7

• who entered the study?
• is the sample representative of the source population?
• what is the response rate?

If the sample is not representative of the source population, is the prevalence likely to be over estimated, underestimated or not affected?

Question 8

describe how selection bias works in case-control studies

Answer 8

can be susceptible to selection bias (is one of its limitations)
- when study and comparison groups are selected inappropriately or using different criteria

participants are selected on the basis of their outcome status
- if this selection is in some way dependent on their exposure status then bias can occur
- selection of cases and controls MUST be independent of the exposure status

three steps you can consider:
- who does it affect?
- how does it affect them?
- what effect does it have on the measure of association? (is it under or over estimated? and specify its direction in relation to the null value)

Question 9

describe ways to avoid selection bias in the cases and controls in case-control studies

Answer 9

cases: if the identification or participation of cases is related to exposure status there can be bias
- ensure high participation
- clearly defined population of interest
- reliable way of ascertaining all cases or a representative sample of cases

controls:
- ensure controls are for the same defined population as the cases over the same time period
- same inclusion and exclusion criteria for cases and controls
- ensure high participation

Question 10

describe how selection bias works in cohort studies studies

Answer 10

have potential for loss to follow up in its limitations (which can lead to selection bias)
- if loss to follow up is related to both the exposure and the outcome this can lead to bias
- if comparison group selected separately from exposed group this can lead to bias too (eg. healthy worker effect)

Question 11

describe how selection bias works in randomised controlled trials

Answer 11

have potential for loss to follow up in its limitations (which can lead to selection bias)
- if loss to follow up is related to both the exposure and the outcome this can lead to bias
- also important to have concealment of allocation

Question 12

three main points/tips when considering selection bias

Answer 12

• consider who enters the study and who stays in the study
• different study designs are susceptible to bias in different ways
• using GATE can help you tract what is happening in a study