Question 1

1000 adults who presented to their local emergency department with a possible heart attack had a blood sample collected for laboratory tests.It was later found that 300 of
These people had not had a heart attack and the levels of triglycerides in the blood of
This group were compared with the levels among the 700 who had had a heart attack
Giving the following results:  The odds ratio for the association between high triglycerides and heart attack is:

Accepted Answer

Odds ratio = (600 x 200) ÷ (100 x 100) = 12.0.This suggests that people with
high levels of triglycerides in their blood are 12.0 times as likely to have a heart attack
compared to those with low levels.The odds ratio is the most appropriate measure of
association for a case‐control study where participants are selected based on their
outcome status (i.e.having had a heart attack or not having had a heart attack).

Question 2

In the first 12 years of follow‐up of the Framingham Heart Study, the observed number of cases of angina was 1.6 times higher than the number expected based on
Population rates.What type of measure is this?

Accepted Answer

It is a standardised incidence (or morbidity) ratio.The key here is the fact
that the observed number of cases was compared to the number of cases expected to
have occurred if the Framingham cohort had had the same incidence of angina as the
general population.An SIR of 1.0 indicates that the occurrence in the study population
did not differ from the general population; an SIR >1.0 indicates that more cases of
disease occurred in the study population than expected while an SIR <1.0 shows fewer
cases occurred than expected.

Question 3

A randomised, placebo‐controlled trial was conducted in Indonesia to study the effects of vitamin A in preventing deaths among children with measles.The investigators
Reported a relative risk of 0.60 for the intervention versus control group.This means
That:

Accepted Answer

Children receiving vitamin A were 40% less likely to die from measles than
children receiving the placebo.This is also known as the Relative Risk Reduction (RRR)
which tells us how much vitamin A reduced the risk of death from measles in the
intervention group as compared to the control group.It is calculated as 1.0 - RR = 1.0 -
0.6 = 0.4 or 40%.The RR of 0.6 tells us that children who received vitamin A were 0.6
times as likely to die from measles compared to children who did not receive vitamin A.

Question 4

Which of the following factors will influence the population attributable fraction? (Select all that apply)

Accepted Answer

The population attributable fraction (PAF) depends on both the
strength of the association between the exposure and the disease measured by the
relative risk and on the prevalence of the exposure in the population.The higher these
are the higher the PAF will be.As the PAF measures the proportion of cases attributable
to the exposure it does not make a difference how common the disease is (B and D).

Question 5

The following table shows data from an epidemiological study.What is the rate difference?

Accepted Answer

Rate difference = Incidence rate in the exposed group - Incidence rate in the
unexposed group:
IR (exposed) = 700 ÷ 1950 py = 35.9 per 100 py
IR (unexposed) = 300 ÷ 2250 py = 13.3 per 100 py
Rate difference = 35.9 per 100 py - 13.3 per 100 py = 22.6 per 100 py

Question 6

In a study to determine whether tonsillectomy is associated with subsequent development of Hodgkin's disease, the estimated relative risk for those with prior
Tonsillectomy was found to be 1.9.From this we can conclude:

Accepted Answer

The incidence of Hodgkin's disease is higher among those who have had a
prior tonsillectomy than among those who have not had a tonsillectomy.We cannot
make any claims about the case fatality rate from this information (A).As the relative
risk is greater than 1.0 for those who have had tonsillectomy it does not appear that
tonsillectomy protects against Hodgkin's disease, if anything the opposite appears to be
true (C).We would want much more information before concluding that tonsillectomy
was causally related to Hodgkin's disease (D).Furthermore, even if the relation were
causal, we would still need to balance the potentially large benefits of tonsillectomy
against the possible small increase in risk of a rare disease before deciding whether the
procedure should be performed.

Question 7

What is the population attributable risk in the same study?

Accepted Answer

PAR = Incidence rate in the whole population - Incidence rate in the
unexposed group
IR (population) = 1000 ÷ 4200 py = 23.8 per 100 py
IR (unexposed) = 300 ÷ 2250 py = 13.3 per 100 py
PAR = 23.8 per 100 py ‐ 13.3 per 100 py = 10.5 per 100 py

Question 8

Which of the following measures is most useful for assessing the potential benefits of a preventive programme?

Accepted Answer

The population attributable fraction (PAF) estimates the proportion of cases
in a population that can be attributed to a particular risk factor; it therefore represents
the maximum percentage reduction in the burden of disease that might be expected if
we could remove the exposure completely.Each of the other measures provides some
useful information about the disease and the potential for intervention, but the most
complete picture is given by the PAF.

Question 9

To assess the association between Kawasaki syndrome (KS) and carpet shampoo, investigators conducted a case‐control study with 100 cases (children with KS) and 100
Controls (children without KS).Among the children with KS, 50 had a history of recent
Exposure to carpet shampoo.Among the controls, the number with a recent history of
Exposure to carpet shampoo was 25.For this study, the odds ratio was:

Accepted Answer

OR = (50x75) ÷ (50x25) = 3.0.This suggests that people exposed to carpet
shampoo are 3 times as likely to develop Kawasaki syndrome than those who are not
exposed.

Question 10

The strength of an association between exposure and disease is best measured by the:

Accepted Answer

Relative risk: this tells us how many times more likely it is that someone who
is exposed to something will develop a certain disease or experience a particular health
outcome than someone who is not exposed.It provides information about the strength
of association between exposure and outcome.The incidence rate (A) measures the
amount of disease in a single group; on its own it tells us nothing about the strength of
association between exposure and disease.The attributable risk (B) tells us how much
more disease is occurring in an exposed group than in an unexposed group and the
attributable fraction (C) tells us the proportion of disease in an exposed group that can
be attributed to the exposure, if we think the association is causal.Neither tells us
directly about the strength of the association between exposure and disease.The
population attributable risk (D) tells us how much more disease is occurring in a
population than would be expected if no‐one in the population was exposed to the
factor of interest.

Quiz 5: Why Linking Exposure and Disease

In the first 12 years of follow‐up of the Framingham Heart Study, the observed number of cases of angina was 1.6 times higher than the number expected based on Population rates.What type of measure is this?

A randomised, placebo‐controlled trial was conducted in Indonesia to study the effects of vitamin A in preventing deaths among children with measles.The investigators Reported a relative risk of 0.60 for the intervention versus control group.This means That:

Which of the following factors will influence the population attributable fraction? (Select all that apply)

The following table shows data from an epidemiological study.What is the rate difference?

In a study to determine whether tonsillectomy is associated with subsequent development of Hodgkin's disease, the estimated relative risk for those with prior Tonsillectomy was found to be 1.9.From this we can conclude:

What is the population attributable risk in the same study?

Which of the following measures is most useful for assessing the potential benefits of a preventive programme?

The strength of an association between exposure and disease is best measured by the: