Lecture 25: Selection Bias

Question 1

What is chance?

Answer 1

chance is random sampling error

Question 2

How can we reduce random sampling error/chance?

Answer 2

by increasing the sample size

Question 3

What are the four potential explanations for a study’s findings?

Answer 3

• it could be a true association
• it could be due to chance
• it could be due to bias
• it could be due to confounding

Question 4

Define bias

Answer 4

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

Question 5

What is systematic error and how does it differ from chance?

Answer 5

it is error that is not random (like chance is) and it has a pattern (unlike chance does)

Question 6

Why does bias matter?

Answer 6

because it can result in an incorrect estimate of the association between exposure and risk of disease

Question 7

Systematic error can mean that the findings could be (3)?

Answer 7

• over estimated
• under estimated
• not affected

Question 8

What are the three potential sources of bias?

Answer 8

• selection bias
• information bias
• publication bias

Question 9

When can we control for selection and information bias?

Answer 9

in the design and data collection stages

Question 10

What can the study investigators do in the planning stages of the study?

Answer 10

• identify the potential sources of bias in the proposed study
• identify possible ways to minimise these potential sources of bias

Question 11

When does selection bias occur?

Answer 11

it occurs when there is a systematic difference between the people who are included in the study and those who are not, or when the study and comparison groups are selected inappropriately or using different criteria

Question 12

What three things do we need to think about when considering selection bias?

Answer 12

• how do we recruit people into epidemiological studies?
• does everyone agree to participate?
• does everyone remain in the study?

Question 13

Why do we need to think about how we recruit people into the study? How should we recruit people?

Answer 13

• if we ask for volunteers, the people that volunteer might be different from those who don’t (eg. they might be healthier of have an interest in the condition)
• we should randomly select people from a defined population in order to be more representative

Question 14

How does bias relate to “does everyone agree to participate”?

Answer 14

not everyone is going to agree to participate and if people who are agree are different from the people that do not agree then bias can occur and this can affect the findings of the study

Question 15

How does bias relate to “does everyone remain in the study”?

Answer 15

because if the reason why people stop taking part is due to the exposure or outcomes, then bias can occur

Question 16

Why would people stop taking part in the study (5)?

Answer 16

because the study goes for a long time, people could lose interest, they could move away, they could be too sick or could die

Question 17

We want to reduce loss to follow up as much as possible. What are some ways to do this?

Answer 17

• having alternative contact details
• maintaining regular contact with patients
• doing multiple attempts to get a response

Question 18

How can selection bias affect a cross sectional study?

Answer 18

because CSS only look at exposures and outcomes at one point in time, bias associated with loss to follow up is not a problem but who enters the study is still relevant

Question 19

What three things do we need to ask when we are considering selection bias in a CSS?

Answer 19

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

Question 20

If the sample was not representative of the source population, we need to consider whether the prevalence is like to be (3)

Answer 20

• over estimated
• under estimated
• not affected

Question 21

One of the big limitations of case-control studies is that they can be susceptible to selection bias. How can this be?

Answer 21

Case control studies occur when you recruit people with the outcome as your cases and people without the outcome as the controls and trace backwards to see if either group was exposed to the outcome. Because selection bias can occur when study or comparison groups are selected inappropriately or using different criteria, it is very susceptible to selection bias

Question 22

How can selection bias occur in case-control studies?

Answer 22

We know that participants’ selection is automatically related to their outcome because that is the basis on which they are selected. But if their selection based on outcome (either the way they are recruited of whether they actually participated) also related to the exposure status, then bias can occur

Question 23

In CCS, selection of cases and control must be independent of the ________ _________

Answer 23

exposure status

Question 24

What is the function of the control group in a case-control study?

Answer 24

to provide an estimate of the odds of exposure in the population from which the cases came

Question 25

As well as considering the selection of cases in a CCS, we also need to consider the selection of ________

Answer 25

controls

Question 26

What are three potential biases in the selection of controls in a CCS?

Answer 26

- if the controls are not representative of the population which gave rise to the cases (in place or time) - if the inclusion/exclusion criteria differs between cases and controls - if there is non-participation of eligible controls

Question 27

What can we do to eliminate "if the controls are not representative of the population which gave rise to the cases (in place or time)" as a potential source of bias in CCS?

Answer 27

ensure the controls are from the same defined population as the cases over the same time period

Question 28

What can we do to eliminate "if the inclusion/exclusion criteria differs between cases and controls)" as a potential source of bias in CCS?

Answer 28

have the same inclusion and exclusion criteria for both cases and controls

Question 29

What can we do to eliminate "if there is non-participation of eligible controls" as a potential source of bias in CCS?

Answer 29

ensure there is high participation

Question 30

How do we calculate the odds ratio of a CCS?

Answer 30

the number of cases in the exposed group divided by the number of cases in the comparison group all divided by the number of controls (no cases) in the exposed group divided by the number of controls (no cases) in the comparison group (no exposure) (a/c)/(b/d)

Question 31

What are the three steps involved in the general approach to the effects of bias?

Answer 31

- who does it affect? - how does it affect them? - what effect does this have on the measure of association - is it under or overestimated?

Question 32

For a harmful factor, is the relative risk above or below the null value?

Answer 32

above

Question 33

For a protective factor, is the relative risk above or below the null value?

Answer 33

below

Question 34

What can we say if the MOA is underestimated for a harmful factor?

Answer 34

that the MOA is biased numerically downwards, or it's biased towards the null.

Question 35

What can we say if the MOA is underestimated for a protective factor?

Answer 35

it is biased numerically upwards, or it's biased towards the null

Question 36

One of the big limitations of cohort studies is that they can be susceptible to selection bias. How can this be?

Answer 36

Cohort studies occur when you recruit people that have had an exposure and then you follow up over time to see if they develop the outcome or not. This means that there is a huge potential for loss to follow up in a cohort study and therefore they are susceptible to selection bias

Question 37

How can loss to follow up cause selection bias in a cohort study?

Answer 37

if the loss to follow up is related to both the exposure and the outcome, this can lead to bias

Question 38

How does the healthy worker affect relate to bias in a cohort study?

Answer 38

In a cohort study, one group is selected and they are classified on whether they have the outcome or not. However, sometimes the comparison group can be selected separately from the exposed group. If they are selected separately, this can lead to bias. HWE is an example of this: if the workers are the exposed group then people who do not work are the comparison group and they may have different characteristics compared to the working population because the non-working population includes people that are too unwell to work

Question 39

One of the big limitations of RCTs is that they can be susceptible to selection bias. How can this be?

Answer 39

In an RCT, participants are randomly assigned to an intervention or control group and then they are followed up over time to see if they developed the outcome. Therefore there is the potential for loss to follow up which is a factor in selection bias