Critical Thinking - Lecture Twenty-Five

Question 1

Bias

Answer 1

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

Question 2

Findings of systematic error

Answer 2

Over-estimation, under-estimation or not affected

Question 3

When is systematic error important?

Answer 3

When critically appraising scientific literature, in evidence-based practice, when considering studies reported in the media and undertaking research

Question 4

What can be done about bias?

Answer 4

Selection and information bias can only be controlled during the design and data collection phases of a study

Question 5

What must investigators do during planning stages of a study?

Answer 5

Identify the potential sources of bias in the proposed study and the possible ways to minimise these potential biases

Question 6

Selection bias

Answer 6

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

Question 7

Three important questions when planning stages in a study

Answer 7

How do we recruit people into epidemiological studies?
Does everyone agree to participate?
Does everyone remain in the study?

Question 8

Volunteering

Answer 8

People who volunteer might be different to people who don’t volunteer and cause bias

Question 9

Representative sample

Answer 9

Randomly select people from a defined population that they’re interested in

Question 10

Selection bias and different study designs

Answer 10

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

Question 11

Cross-sectional studies

Answer 11

Exposures and/or outcomes assessed at one point in time, and measuring prevalence

Question 12

Considering selection bias

Answer 12

Who entered the study?, is the sample representative of the source population?, what is the response rate?

Question 13

If the sample was not representative of the source population is the prevalence likely to be

Answer 13

Over-estimated?
Under-estimated?
Not affected?

Question 14

Selection bias: case-control studies

Answer 14

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

Question 15

How are participants selected in the case-control studies

Answer 15

On the basis of their outcome status

Question 16

How can bias occur in terms of selection for case-control studies?

Answer 16

If the selection is dependant on their exposure status

Question 17

How to avoid potential biases in selection of cases

Answer 17

Ensure high participation
Clearly defined population of interest
Reliable way of ascertaining all cases or a representative sample of cases

Question 18

Potential biases in selection of controls

Answer 18

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

Question 19

Controls not representative of the population which gave rise to cases (in place or time)

Answer 19

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

Question 20

If inclusion/exclusion criteria differ between cases and controls

Answer 20

Same inclusion and exclusion criteria for cases and controls

Question 21

If non-participation of eligible controls

Answer 21

Ensure high participation

Question 22

Selection bias in loss of follow up

Answer 22

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

Question 23

How can the selection of comparison and exposed groups cause bias?

Answer 23

When they are selected separately