Research Methods - Representing risk

Question 1

What is incidence?

Answer 1

Incidence measures the frequency of “new onset cases” of a disease (/condition/outcome) (E.g. COVID-19) in a given population over a designated period of time (e.g. 12 months)

Question 2

Incidence is expressed as a … of a … which is at risk

Answer 2

Incidence is expressed as a proportion of a population which is at risk

Question 3

Incidence is the number of people who develop a disease over …

Answer 3

number of people who develop disease over given time

Question 4

What it person-time?

Answer 4

Person years (or person months) are measurements based on the number of people in the study and the amount of time each person spends in the study

Question 5

Person-time assumes disease probability during study period is …, so that 10 persons/1 year = 1person/10 years

Answer 5

Person-time assumes disease probability during study period is constant, so that 10 persons/1 year = 1person/10 years

Question 6

Person-time is often invalid - why?

Answer 6

As risk of many chronic diseases increases with age

Question 7

What is the relationship between incidence and prevalence? (Assuming all other factors e.g. disease prognosis stay constant)

Answer 7

As incidence increases, prevalence increases

Question 8

Prevalence = …. x (duration of disease)

Answer 8

Prevalence = Incidence x (duration of disease)

Question 9

The epidemiologist’s bathtub

Answer 9

Question 10

The relationship between prevalence and incidence

Answer 10

Question 11

How we might we find data to measure incidence?

Answer 11

Cohort study - groups of people who have something in common

Followed up of over time to see what happens to them

Question 12

What is a cohort study?

Answer 12

Group of individuals free from a disease or condition is selected, ideally at random
Typically an exposure of interest - participants selected into an exposed and a non-exposed group

Question 13

In a cohort study, no allocation/manipulation of exposure by the researcher - but groups may be matched on potential … factors

Answer 13

Confounding factors i.e. influential factors not of interest

Question 14

How is a cohort study conducted?

Answer 14

The groups are followed up over the given period of time - occurrences of disease onset are recorded
At the end of the study, comparisons are made between the non/exposed groups and their relative numbers of new disease occurrences

Question 15

What is a cohort study - Prospective vs Retrospective

Answer 15

Prospective = exposed and non-exposed groups, outcomes followed up in the future
Retrospective = Historical exposed and non-exposed groups, outcomes followed over defined historical period

Question 16

… = exposed and non-exposed groups, outcomes followed up in the future
…. = Historical exposed and non-exposed groups, outcomes followed over defined historical period

Question 17

What type of epidemiological study is a cohort study? Descriptive or analytic?

Answer 17

Analytic (tests hypothesis, identify and quantify risk or exposure)

Question 18

… cohort = secondary data only

Answer 18

Retrospective cohort = secondary data only

Question 19

… cohort = any/all data sources as relevant

Answer 19

Prospective cohort = any/all data sources as relevant

Question 20

Cohort study data: Secondary (mortality register, hospital/medical records, census data)
Pros vs Cons

Answer 20

+ Cheap
+ If anonymous, minimal ethical/governance approval needed
- Limited by what data already gathered
- Poor accuracy and missing data

Question 21

Cohort study data: Primary (survey data)

Pro vs cons

Answer 21

+ Gather additional data

• Difficult to achieve representative sample
• More expensive

Question 22

Strengths of the cohort study (4)

Answer 22

More than one disease (outcome) related to single exposure
Can offer some evidence of cause - effect relationship through establishing temporal precedence
Good when exposure is rare
Can calculate incidence rate and risk

Question 23

Weaknesses of the cohort study:

Answer 23

Potential for losses to follow-up (attrition may differ in exposed versus unexposed/disease versus non, the longer the follow-up the greater the risk of attrition)
Often requires large sample, long duration, expensive
Less suitable for rare diseases
Less suitable for diseases with long latency
Does not eliminate confounding

Question 24

Calculating … = number of people who develop disease over a given time / number of people at risk of developing disease over given time

Answer 24

Calculating incidence = number of people who develop disease over a given time / number of people at risk of developing disease over given time

Question 25

Measuring incidence - relative risk:

A clear advantage of the cohort study is that we can compare incidence between the two groups i.e. to test hypotheses about associations between exposures and outcomes
Relative risk or risk ratio (RR) is the risk of developing a disease in … group compared to developing a disease in …. group

Answer 25

A clear advantage of the cohort study is that we can compare incidence between the two groups i.e. to test hypotheses about associations between exposures and outcomes
Relative risk or risk ratio (RR) is the risk of developing a disease in exposed group compared to developing a disease in unexposed group

Question 26

… = incidence of disease among exposed / incidence of disease among non-exposed

Answer 26

Relative risk

Question 27

Cohort study data analysis: Relative risk

Answer 27

Question 28

RR of 1.5 means the risk of outcome is … higher in the … group than the … group

Answer 28

Risk of outcome 50% higher in exposed than unexposed group

Question 29

RR of 3.0 means the risk of outcome is … higher in the … group than the … group

Answer 29

RR of 3.0 means the risk of outcome is 3 times higher in the exposed group than unexposed group

Question 30

RR of 0.8 means the risk of outcome is … higher in the … group than the … group

Answer 30

Risk of outcome 20% lower in exposed than unexposed group

Question 31

Calculating incidence - disease incidence

Incidence of disease = (…+…)/N
Incidence of disease in exposed = …/(…+….)
Incidence of disease in unexposed = …/(…+…)

Answer 31

Incidence of disease = (A+C)/N
Incidence of disease in exposed = A/(A+B)
Incidence of disease in unexposed = C/(C+D)

Question 32

Relative risk of 1.94 for risk of tuberculosis for homeless versus non-homeless people in London - what do we conclude?

Answer 32

Homeless people in London have a greater risk of developing tuberculosis

Question 33

The same relative risk can represent completely different absolute differences in how many people are affected by a disease/die -
depending on absolute prevalence in the population

Rare disease/outcome = … RR not important for public health (important to the individual)
Common disease/outcome = … RR clinically important in public health

Researchers should report both RR and absolute risk

Answer 33

The same relative risk can represent completely different absolute differences in how many people are affected by a disease/die -
depending on absolute prevalence in the population

Rare disease/outcome = large RR not important for public health (important to the individual)
Common disease/outcome = small RR clinically important in public health

Researchers should report both RR and absolute risk

Question 34

…. …. - precision of our estimates e.g. for people in London, homelessness is associated with relative risk of contracting TB of 1.94 - how confident are we that this 1.94 is precise? -

Answer 34

The confidence interval describes the range of values with a given probability (e.g. 95%) that the true value of a variable is contained within that range.

Question 35

The … … describes the range of values with a given probability (e.g. 95%) that the true value of a variable is contained within that range.

Answer 35

The confidence interval describes the range of values with a given probability (e.g. 95%) that the true value of a variable is contained within that range.

Question 36

Relative Risk or Risk Ratio (RR) is the risk of developing a disease in … group compared to developing a disease in … group

Answer 36

Relative Risk or Risk Ratio (RR) is the risk of developing a disease in exposed group compared to developing a disease in unexposed group

Question 37

Confidence intervals = a way of expressing the precision of …
Compared to the likelihood ‘…’ value in the population

Answer 37

Confidence intervals = a way of expressing the precision of estimates
Compared to the likelihood ‘true’ value in the population

Question 38

…. study = select exposed and unexposed individuals at outset and measure incidence of outcome over given period of time

Answer 38

Cohort study = select exposed and unexposed individuals at outset and measure incidence of outcome over given period of time

Question 39

Pros of cohort study

Answer 39

Advantages – can establish temporal precedence of exposure to outcome, suitable for rare exposures

Question 40

Cons of cohort study

Answer 40

Disadvantages – does not eliminate confounding, less suitable for rare diseases/outcomes