Study on the Long-term Effect of Physical Activity on Incidence of Coronary Heart Disease and a Case-Control Study on the Effect of Dietary Fat Intake on Melanoma

The University of Western Sydney
School of Science and Health
Introduction to Epidemiology (401076)
ASSIGNMENT 2
Spring, 2018

Based on the following study
Sundquist K., Qvist J. Johansson SE., Sundquist J. The long-term effect of physical activity
on incidence of coronary heart disease: a 12-year follow-up study Preventive Medicine
2005;41:219-225
Question 1
(a) The study design employed in the study [1 mark]
- This is a cohort study utilized prospective study. The outcomes were assessed as
the study progressed and outcomes measured at the end of the study.
(b) Data sources for coronary heart diseases in the study [2 marks]
- Swedish national discharge register data source was used to identify the events
coronary heart disease from the respondents.
(c) Why the authors excluded 1894 participants who rated their general health as “poor”
[2 marks]
- The authors excluded these participants as it would have affected the study
because of its confounding effects on the issue of interest in the study which was
coronary heart disease emergence. Having respondents with underlying or poor
health conditions could lead to coronary heart disease being linked to other
causative factors part from the issue of interest which is the respondent’s physical
activity.
(d) The crude overall CHD incidence in the study [2 marks]
- The overall crude incidence rate was 59 cases per 10,000 persons over physical
activity levels.
(e) Comparison of physical activity being undertaken at least twice a week for low
income and all other income categories [4 marks]
- The risks obtained at 95% confidence level reveals that physical activity of twice
daily had RR of 0.72 (CI 0.52-1.01) compared to low-income earners having RR
of 1.20 (0.95-1.52) and those of all other income earners at RR equals to 1.
(f) The interpretation of the relative risk above [3 marks]
- The relative risks above illustrate that exposure to undertaking physical activity
twice a week decreased the exposure of developing coronary heart disease.
- Low income was further associated with the risk of developing coronary heart
disease, thus being a low-income earner had a greater risk of developing coronary
heart disease, while all other income earners, in general, had no effect on
developing coronary heart disease.
(g) The relative risk in this study is better described as risk ratio in that [2 marks]
- Risk ratio refers to the probability of an outcome in an exposed group to the
probable outcome of the exposed group, while rate ratio refers to comparing rates
of events taking place at any given time. This study is best explained when risks
ratio is assessed on the risks of developing coronary heart disease to factors such
as physical activity, income, smoking and BMI assessments for the respondents.

In this case the strength of the association between the risks factors and outcome
of developing coronary heart disease.
(h) The major confounders in this study which were not included include the following
[4 marks]
- Some of the major confounders which were not assessed in this study include
hyperlipidemia, which has been considered as a powerful indicator of coronary
heart disease, with the positive association being linked to cholesterol levels.
Further hypertension could be a major confounding fact in the development of
coronary heart disease. Hypertension has been attributed as a strong and
independent risk factor for the development of coronary heart disease causing
morbidity and mortality.
- Further diabetes mellitus could a major confounding factor. Both type 1 and 2,
have been associated with elevated risks for developing heart disease. Thus it
could act as a confounding factor in the development of heart disease.
(i) These confounders were not adjusted due to the following reasons [4 marks]
- The confounding factors were not added into the study due to the longitudinal
nature of the study, which followed the group cohorts over 10 years, in this case,
effects of variables under study was sought.
- The study was designed in such a way that those respondents who attributed their
health state being poor or bad were excluded in the participation. Hence those
having poor self-rating of their disease condition were not included in the study,
and the study only focused on those who had good health status.
Question 2:
Tasmanian researchers conducted a case-control study to investigate the effect of dietary fat
intake on melanoma (Skin Cancer). They hypothesized that people whose dietary fat intake is
low will be more susceptible to skin cancer. The study compared 500 cases of melanoma
with 500 controls. The controls were randomly selected from the state’s electoral roll. The
researchers categorized the dietary fat intake into three categories High, Moderate and Low.
They found that among skin cancer patients 150 were classified at the low and 80 at the high
dietary fat intake whereas among control 130 were at the low and 100 were at the high.
a) Summarised table for estimation of the level of association of the diseases and fat
intake [4 points]
Having disease The absence of the
disease
Total
High fat intake 80 100 180
Moderate fat
intake
270 270 540
Low fat intake 150 130 280
Total 500 500 1000