interpreting epidemiological findings

Question 1

what are Koch’s postulates used for?

Answer 1

infer whether a particular pathogen is responsible for a particular disease

Question 2

what do Koch’s postulates state?

Answer 2

microorganism must be found in abundance in all organism suffering from the disease but not in healthy organisms

must be isolated from a diseased organism and grown in pure culture

cultured microorganism should cause disease when introduced into a healthy organism

must be re-isolated from the inoculated diseased, experimental host and identified as being identical to the original specific causative agent

Question 3

why do Koch’s postulates sometimes fail?

Answer 3

unethical to re-introduce microorganisms into humans with the intention of causing infection

Question 4

what does the Bradford-Hill criteria allow?

Answer 4

infer causation from observational and interventional methods

Question 5

what are the Bradford-Hill criteria?

Answer 5

strength

consistency

specificity

temporality

biological gradient

plausibility

coherence

experiment

analogy

Question 6

what does ‘strength’ mean with reference to the Bradford-Hill criteria?

Answer 6

strong relationship between an exposure and an outcome makes it more likely that there may be a causative element

Question 7

what does ‘consistency’ mean with reference to the Bradford-Hill criteria?

Answer 7

stronger association increases confidence that an exposure causes an outcome

consistent findings across settings can rule out errors/biases that may take place in 1 or 2 studies

Question 8

what does ‘specificity’ mean with reference to the Bradford-Hill criteria?

Answer 8

specificity - describes association between specific causes and effects

specific disease arising amongst specific workers supports argument for causality

Question 9

what are the criticisms of ‘specificity’ in the Bradford-Hill criteria?

Answer 9

Hill concedes that lack of specificity doesn’t necessarily invalidate the relationship

specificity is difficult to account for in multifactorial issues as many factors interact

therefore specificity is informative when present, but absence of specificity does not mean much

Question 10

what does ‘temporality’ mean with reference to the Bradford-Hill criteria?

Answer 10

exposure must commonly precede an outcome

Question 11

which study designs are better to assess ‘temporality’ with reference to the Bradford-Hill criteria?

Answer 11

cross sectional studies - look at many people at same point in time to assess presence of exposure and outcome, cannot assess which preceded which

longitudinal studies can assess temporality better

Question 12

what does ‘biological gradient’ mean with reference to the Bradford-Hill criteria?

Answer 12

dose-response effect in the ‘right’ direction suggests a relationship between an exposure and outcome

(e.g. higher risk of CVD caused by smoking - the greater the number of cigarettes smoked, the greater the risk)

Question 13

what are the criticisms of ‘biological gradient’ in the Bradford-Hill criteria?

Answer 13

quantification is difficult

when present it is suggestive but absence of biological gradient has little value

Question 14

what does ‘biological plausibility’ mean with reference to the Bradford-Hill criteria?

Answer 14

relationship is biologically plausible where the science is understood

Question 15

how does ‘biological plausibility’ (Bradford-Hill) change over time?

Answer 15

advances in scientific understanding allows previously implausible relationships to be explained

Question 16

what does ‘coherence’ mean with reference to the Bradford-Hill criteria?

Answer 16

association is consistent with existing theory and knowledge

slight overlap with biological plausibility

Question 17

what does ‘experiment’ mean with reference to the Bradford-Hill criteria?

Answer 17

evidence from experimentation is supportive of proposed link

Question 18

what is the issue with ‘experiment’ (Bradford-Hill criteria)?

Answer 18

experimentation within ethical constraints is difficult

Question 19

what does ‘analogy’ mean with reference to the Bradford-Hill criteria?

Answer 19

analogous findings may provide inferences about relationships

e.g. rubella causes deafness, eye problems, heart problems - by analogy other viral illnesses contracted during pregnancy may cause similar pathology

Question 20

what is the issue with ‘analogy’ (Bradford-Hill criteria)?

Answer 20

depends on existing knowledge

Question 21

what is the purpose of validity and bias?

Answer 21

determine whether the results of a study are relevant and should be trusted or not

Question 22

what is internal validity?

Answer 22

association exists amongst a group of patients within a study

observed results are due to true cause-and-effect

Question 23

what is external validity/generalisability?

Answer 23

association determined in source population (internal validity) also applies outside of source population - in different places or times etc.

Question 24

what is the importance of external validity?

Answer 24

determines real-life impact of a finding

Question 25

what is bias?

Answer 25

any trend in the collection, analysis, interpretation, publication or review of data that can lead to conclusions that are systematically different from the truth

Question 26

what is the difference between random and systematic error?

Answer 26

random:

• occasional underestimation/overestimation due to chance
• interferes with precision
• errors can be more or less cancelled out with a large enough sample size

systematic:

• consistent underestimation/overestimation of measurements
• significantly alters distribution pattern
• introduces bias

Question 27

what are the 3 types of bias?

Answer 27

selection bias

information bias

confounding

Question 28

what is selection bias?

Answer 28

for results to be valid sample must be representative of study population

however, an individual’s chance of being included in the study sample may be related to both exposure and outcome

causes biased estimate of association between exposure and outcome

Question 29

what is Berkson’s bias?

Answer 29

type of selection bias

hospital based case control study where controls are selected amongst the hospital’s patients

Question 30

what is the healthy worker effect?

Answer 30

type of selection bias

active workers are more likely to be healthy compared to those who have retired or stopped working

Question 31

what are 3 ways of minimising selection bias?

Answer 31

select controls which are representative of the study population in terms of exposure to the facts on the study

minimise non-response (if people decline to participate it could introduce bias)

compare those included in the sample with those who declined to respond and explore whether there are any systematic differences.

Question 32

what is information bias?

Answer 32

misclassification of exposure, disease status or both

Question 33

why can information bias occur?

Answer 33

study variables not properly defined

flaws in data collection

Question 34

what is interview bias?

Answer 34

common flaw in data collection that causes information bias

interviewers ask individuals about their exposure status but may be more or less thorough depending on their outcome

causes misclassification of exposure status and eventually biased odds ratio

Question 35

how can interview bias be prevented?

Answer 35

interviewer does not know individual’s disease status

collection process has
been carefully standardised so that interviews follow a strictly defined protocol when collecting participant data

Question 36

what is recall bias?

Answer 36

causes information bias

refers to differentially inaccurate recall of past exposure between cases and controls

patients are more likely to recall a brief exposure if they have developed the outcome

Question 37

how can recall bias be prevented?

Answer 37

use objective ways to assess exposure

e.g. medical records, biomarkers

Question 38

what is non-differential misclassification?

Answer 38

exposure status is misclassified but equally so amongst cases and controls

e.g. all participants have trouble remembering that exposure status - has nothing to do with their disease (not recall bias)

or - there are errors in determining outcome but they occur equally in exposed and non-exposed individuals

Question 39

what type of bias does non-differential misclassification result in?

Answer 39

odds ratio biased towards null

Question 40

what is differential misclassification?

Answer 40

errors in determining an individual’s exposure status occur unevenly amongst cases and controls

or - errors in the diagnosis of the disease occur unevenly amongst exposed and unexposed individuals

Question 41

what type of bias does non-differential misclassification result in?

Answer 41

biased odds ratio

cannot predict if it is biased towards or away from null