WEEK 4: measures of association, measures of public health impact & errors

Question 1

Weaknesses of an ecological study?

Answer 1

• Ascertainment of the disease: quality of recording across time/place can vary
• Idem info on exposure
• Data obtained on non-representative samples
• Often no adjustments for confounders apart from age, sex
• Not good for rare exposures, because there is no good reliable data for this across countries

Question 2

Which measure of association: RCT?

Answer 2

Incidence in exposed vs unexposed and compare with each other: incidence proportion ratio or incidence rate ratio

Question 3

Which measure of association: Cross-sectional study?

Answer 3

Use prevalence proportion ratio to ‘count’ the cases (prevalence in exp/prevalence in unexp)

Question 4

Which measure of association: Case-control?

Answer 4

Odds ratio = ad/bc (a = exposed)

(Exp diseased x not exp not diseased)/(exp not diseased x not exp diseased)

Question 5

Which measure of association: cohort?

Answer 5

relative risk (incidence proportion ratio or incidence rate ratio)

IRR: IR in exp/IR in unexp
IPR: IP in exp/IP in unexp

Question 6

Why should the cases in a case-control study be incidence cases?

Answer 6

 Survivor bias: if you do not choose the incidence cases and your disease is severe, you only look at prevalent cases that already have survived for a while. These are the less serious cases which results in a bias.

Question 7

Examples of quasi-experimental study?

Answer 7

Example of quasi-experimental study = pre-post design, or intervention in one city, control in the other
Quasi-experimental study can also be (besides no randomization) an intervention that has not been introduced by the researcher or no control group

Question 8

What is the background risk?

Answer 8

The natural occurrence of
disease in the unexposed population

Question 9

What is the RD (rate difference)?

Answer 9

= Rate of exposed people (e.g. smoking) – Rate of non-exposed people (no smokers)

Question 10

RR = ?

Answer 10

Relative risk = Rate exp (e.g. smoking)/Rate non-exposed (e.g. non- smoking)

(1 = no association). Per 100.000

Question 11

What is the RR dependent on?

Answer 11

1. Background risk:
   lower background risk -> stronger RR even when RD (rate difference) is constant
2. Prevalence of other component causes (gene pool)

Question 12

Why do you not use IPR or IRR with a case-control study design?

Answer 12

Incidence is not available, because it is a sample of the population

Question 13

What does the odds ratio mean?

Answer 13

Odds ratio = the odds (not probability) of disease in the exposed relative to the odds of disease in unexposed.

Question 14

What is a major problem in using the odds ratio with case-control studies?

Answer 14

Big problem is confounding (e.g. not adjusting for socio-economic status).

Question 15

What does a RR of < 1 mean?

Answer 15

Negative association, decreased risk or protective effect

Question 16

What is attributable risk? How is it measured?

Answer 16

Measures the actual amount of disease that can be attributed to a particular exposure

Can be
A. Incidence rate difference
B. Incidence proportion difference
C. Prevalence difference

Question 17

What is the difference between attributable and relative risk?

Answer 17

Attributable risk = measures the actual amount of disease
RR = How many times higher is the risk of disease in one group compared to the other?

Question 18

3 different ways to state attributable risk?

Answer 18

● How much extra disease occurs in group A vs B?

or
● What is the excess amount of disease occurring among those
exposed to a risk factor?

or
● How much disease among those who are exposed could
potentially be prevented by removing the exposure?

Question 19

What is the IRD (Incidence rate difference)? Or: excess rate

Answer 19

= IR in exp - IR in unexp

Question 20

What is the IPD (incidence proportion difference)? Or: risk difference/excess risk

Answer 20

= IP exp - IP unexp

Question 21

What is the prevalence difference (PD)? Or: risk difference/excess risk

Answer 21

prevalence proportion exp - prevalence proportion unexp

Question 22

What is the Attributable fraction (AF)?

Answer 22

= Attributable risk/incidence in exp * 100%

= Attributable risk/incidence rate or proportion in exposed * 100%

= (a - b)/a * 100

Question 23

What does the attributable fraction tell you?

Answer 23

-> Tells us the proportion of disease in those exposed that can be attributed to the exposure

Question 24

2 Differenct ways to state the PAR (population attributable risk)?

Answer 24

● How much extra disease occurs in the population compared to an unexposed group?

or
● How much disease among in the population could potentially be prevented by removing the exposure?

Question 25

How to use PAR with rate, proportion and prevalence?

Answer 25

PAR =

IR population - IR unexp
IP population - IP unexp

Or, using attributable risk and prevalence:
AR x P exposed

Question 26

What is the PAF and how is it measured?

Answer 26

PAF = population attributable fraction

(Pop. attributable risk (PAR)/ incidence in the total population) * 100%

Tells us the PROPORTION of disease in the population that can be attributed to the exposure

Question 27

For what purpose do you use relative risk vs attributable risk?

Answer 27

Relative risk: how many times higher is the risk of disease in group A compared to B
> is used for etiology
> note: high relative risk does not always reflect high impact

Attributable risk: Measures actual amount of disease attributed to an exposure
> used for policy decisions
> funding decisions (prevention programmes)
> Which disease should be targeted first?