Lecture 25 - Selection Bias

Question 1

what are four potential explanations for a study’s finding

Answer 1

• True associations
• Chance
• Bias
• Confounding

Question 2

What is Bias

Answer 2

Any systematic error is an epidemiologic study that results in an incorrect estimate of the association between exposure and risk of disease.

Question 3

Why does bias matter

Answer 3

any systematic error in an epidemiologic study that results in an incorrect estimate of the association between exposure and risk of diease

Question 4

Why is bias important to consider in research

Answer 4

It can cause findings to be overestimated, underestimated, or unaffected, impacting the accuracy of study conclusions

Question 5

Systematic error

Answer 5

Findings could be:
- Over estimated
- Underestimated
- Not affected
Important when:
- Critically appeasing scientific literature
- In evidence-based practice
- Considering studies reported in the media
- Undertaking research

Question 6

What can we do?

Answer 6

• Selection and information bias can only be controlled during the design and data collection phases of a study
• So at the planning stages of a study investigators must:
  1. Identify the potential sources of bias in the proposed study
  2. Identify possible ways to minimise these potential biases

Question 7

Three main types of biases

Answer 7

1. Selection Bias
2. Information Bias
3. Publication Bias

Question 8

Selection bias

Answer 8

Occurs when there is a systematic difference between 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

Question 9

selection bias and different study designs

Answer 9

• While all analytic studies are subject to bias, each design had particular types to which it is inherently most vulnerable

Question 10

Cross-sectional studies - selection bias

Answer 10

• If the sample was not representative of the source population

Question 11

Case-control studies - selection bias

Answer 11

If cases and controls are selected based on exposure status, it can result in biased estimates of the association between exposure and outcome

Question 12

Preventing bias in case control study

Answer 12

Cases:
- Ensure high participation
- Clearly defined population of interest
- Reliable way of ascertaining all cases or a representative sample of cases
Controls:
- Ensure controls are from 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 13

How can you assess whether a study’s measure of association has been biased?

Answer 13

Determine whether the bias causes the association to be over or underestimated and wether it is biased towards or away from null value

Question 14

e.g interpretations
Study value: 2.3
True Value: 3.5

Answer 14

It has under-estimated the harmful effect of exposure, the OR is biased towards the null

Question 15

Cohort study - selection bias

Answer 15

Loss of follow up:
- 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 exposure group can lead to bias e.g. healthy worker effect

Question 16

RCT - selection bias

Answer 16

The use of randomisation to evenly distribute potential confounders across intervention and controls groups, reducing bias