lecture 24

Question 1

what is the definition of bias

Answer 1

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

Question 2

in systematic error findings could be ….

Answer 2

• over estimated
• under estimated
• not affected

Question 3

in what phases can selection and information bias be controlled

Answer 3

during the design and data collection phases of study

Question 4

what are the three potential sources of bias

Answer 4

• selection bias
• information bias
• publication bias

Question 5

what is selection bias

Answer 5

when there is a systematic difference between the people included in a study and those who are not

or when the study and comparison groups are selected inappropriately or using different criteria

Question 6

how can 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 people

Question 7

what is possible selection bias in a cross sectional study

Answer 7

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

Question 8

how can bias occur in case control studies in the selection of cases

Answer 8

if the identification or participation is related to exposure status there can be bias

Question 9

what is important about the selection of cases and controls in a case control study

Answer 9

the selection of cases and controls must be independent of their exposure status

Question 10

how to minimise the potential bias that could occur when selecting cases in a case control study

Answer 10

• ensure high participation
• clearly defined population of interest
• reliable way of ascertaining all cases or a representative sample of cases

Question 11

what are some examples of potential bias that could occur when selecting controls for a case control study

Answer 11

• if controls are not representative of the population which gave rise to cases (place and time)
• if inclusion / exclusion criteria differ between cases and controls
• if non participation of eligible controls

Question 12

what are the three steps you can consider about the effects of bias

Answer 12

• who does it affect
• how does it affect them
• what effect does it have on the measure of association

Question 13

what is a harmful factor in relation to bias and has the measure of association been biased towards or away from the null value

Answer 13

• the study found a measure of association that was underestimated
• it is biased numerically downwards OR biased towards the null

Question 14

what is the protective factor in relation to bias and has the measure of association been biased towards or away from the null value

Answer 14

• the study found a measure of association that was overestimated
• it is biased numerically upwards OR biased towards the null

Question 15

what selection bias is possible in cohort studies

Answer 15

• loss to follow up
• if comparison group selected separately from exposed group

Question 16

what selection bias is possible in randomised controlled trials

Answer 16

• if the people that are selecting people into the study knew which group the people were going into
• missing people due to loss to follow up