You are a part of a research team and you have been asked to give advice on the design of a
case-control study to investigate the association between breast cancer and participation in
mammography screening. The research team wants to be able to detect an odds ratio of at least
0.5 using a two-sided significance test at the 5% level. It is estimated that 50% of the control
population will have participated in mammography.
(a) If an equal number of cases and controls could be recruited in this study, what is the
required sample size to achieve a study power of (i) 80% and (ii) 90%?
(b) One of the researchers thinks that the prevalence of the exposure (mammography
screening) in the population would be 30%, not 50%.
i. If the researchers would like to achieve 90% power, how many cases and
controls would be required to detect the same odds ratio as in (a)?
ii. If the prevalence of exposure in the population was not known, would it be
better to assume the prevalence of exposure in the population was 30% or
(c) The study team inform you that they may not be able to recruit as many cases as
you have estimated in (a). What should they do to keep the same power, detectable
effect size, and level of significance as in (a)? What is the disadvantage of this
The Health Department conducted a study to assess the validity of a new field test for lead
exposure, which they wanted to use for a mass screening program. The field test uses a
spot of blood from a finger prick sample which is analysed using a small hand-held machine.
The results from the field test were compared with the gold standard laboratory test for
measuring blood lead concentration in a representative sample of the population identified
for the mass screening program. The gold standard test used a larger blood sample
collected from a vein of each study participant at the same time the finger prick sample was
The results of the study are shown below:
Field test Gold standard
≥ 15 mg/dL 135 100 235
≥ 15 mg/dL 15 1400 1415
Total 150 1500 1650
a) Calculate the sensitivity, specificity, positive predictive value and negative predictive
value of the new field test compared with the gold standard test. Interpret the sensitivity
and the positive predictive value.
b) Is this field-test suitable for use as a screening test? Give reasons that support your
Read the edited extract from a published study shown below. Use the information contained
in the extract to answer the following questions.
(a) What is the research question? Use PICO format
(b) What type of study has been used? Is it the best study design to answer the research
question? Give reasons for your answers.
(c) Describe the source population and the study participants? Were there any issues that
could lead to selection bias? Give reasons for your answer.
(d) What were the study factor and primary outcome factor? What did the investigators do
to minimise measurement error of the outcome factor? Give reasons for your answers.
(e) What is your assessment of the generalisability of the study findings?