Epid6420 Fundamentals Of Epidemiology : Assessment Answers
Questions:
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Question 1
A case control study among 2000 individuals was conducted to determine the association between cigarette smoking and coronary heart disease. Out of the study subjects, 50% had coronary heart disease; 70% of the cases were smokers and 60% of the controls were non-smokers.
1. Draw the 2 x 2 table to explore the relationship between coronary heart disease and cigarette smoking.
2. Calculate the crude odds ratio and explain its exact meaning.
Question 2
A large randomised controlled trial is carried out to determine the effect of pravastatin, a cholesterol lowering medication, in elderly people (>65 years old) at risk of cardiovascular disease, i.e. heart attacks and strokes. Out of 23 770 individuals assessed at various family practices, only 7056 were eligible and agreed to participate. These were entered into a un-in period, in which they were given placebo tablets for 4 weeks. Those who were not compliant in taking their medicine were dropped from the trial, leaving 5804 patients to be randomised.
Answers:
Introduction:
We do not expect, or want you to write a mini essay for each response. Succinct answers which cover all the relevant points are desirable. However, make sure you show the formulae and all workings for any calculations undertaken. This may allow us to still allocate you marks even if your final answer was incorrect (in the unlikely event this would occur!). For all calculations, use up to 2 decimal point.
Question 1
A case control study among 2000 individuals was conducted to determine the association between cigarette smoking and coronary heart disease. Out of the study subjects, 50% had coronary heart disease; 70% of the cases were smokers and 60% of the controls were non-smokers.
1. Draw the 2 x 2 table to explore the relationship between coronary heart disease and cigarette smoking.
|
Groups |
Patient with coronary heart disease |
Control |
Total |
|
Smokers |
700 |
400 |
1100 |
|
Non Smokers |
300 |
600 |
900 |
|
Total |
1000 |
1000 |
2000 |
- Calculate the crude odds ratio and explain its exact meaning. (2 marks)
Crude odds ratio=(Smokers (with CHD) x Nonsmokers (control))/ (Non Smokers (with CHD)x smokers (control))
Crude odds ratio=(700x 600)/ (300x 400)
Crude odds ratio=3.5
The crude odd ratio of the above study is 3.5. If the OR>1, it indicates that smoking is associated with higher odds of coronary heart disease.
Data were further stratified according to gender of the study subjects:
Males
Coronary heart disease
|
Cigarette smoking |
Cases |
Controls |
Total |
|
Exposed |
520 |
140 |
660 |
|
Unexposed |
180 |
360 |
540 |
|
Total |
700 |
500 |
1200 |
OR=(520x360)/(180x140)=7.42
Females
Coronary heart disease
|
Cigarette smoking |
Cases |
Controls |
Total |
|
Exposed |
180 |
260 |
440 |
|
Unexposed |
120 |
240 |
360 |
|
Total |
300 |
500 |
800 |
OR=(180x240)/(120x260)=1.38
Gender is a effect modifier as the effect of smoking in males is greater (OR=7.42) on cardiovascular outcome in comparison to females.
Using this information, explain whether gender is a confounder or effect modifier?
Question 2
A large randomised controlled trial is carried out to determine the effect of pravastatin, a cholesterol lowering medication, in elderly people (>65 years old) at risk of cardiovascular disease, i.e. heart attacks and strokes. Out of 23 770 individuals assessed at various family practices, only 7056 were eligible and agreed to participate. These were entered into a “run-in” period, in which they were given placebo tablets for 4 weeks. Those who were not compliant in taking their medicine were dropped from the trial, leaving 5804 patients to be randomised.
2913 were assigned placebo, and 2891 were assigned pravastatin. Over the next 3 years, 650 and 725 people discontinued their medication, respectively (lack of compliance), but all participants continued to be followed for fatal and non-fatal heart attacks, as well as fatal and non-fatal strokes. The authors perform an intention to treat analysis, with all cardiovascular events being the primary endpoint (combined fatal and non-fatal heart attacks and fatal and non-fatal strokes).
In the placebo group, 473 reached a primary endpoint vs. 408 in the pravastatin group.
1. Draw the 2 x 2 table for all cardiovascular outcomes combined (fatal and non-fatal heart attack and fatal and non-fatal stroke all combined).
|
Groups |
All cardiovascular outcomes combined (fatal and non-fatal heart attack and fatal and non-fatal stroke all combined). |
No cardiovascular Risk |
Total |
|
Pravastatin |
408 |
1758 |
2166 |
|
Placebo |
473 |
1790 |
2263 |
|
Total |
881 |
3548 |
4429 |
- What is the absolute risk of all cardiovascular outcomes in the placebo and drug group? Explain this in words.
AR (absolute risk) = the number of cardiovascular outcomes in pravastatin or placebo groups, divided by the number of people in that group.
Absolute risk of all cardiovascular outcomes in the placebo group
AR (cardiovascular outcomes)= 473/2263=0.20
Absolute risk of all cardiovascular outcomes in the provastatin group
AR (cardiovascular outcomes)= 408/2166=0.18
The absolute risk of developing a cardiovascular disease in placebo is 20%. The treatment with provastatin decreases the absolute risk of cardiovascular disease to 18%. This means, the treatment reduces the absolute risk from 2000 in 10,000 to 1800 in 10,000 indicating that pravastatin reduces the absolute risk in 200 people in every 10,000 people which is significant in case of fatal disease.
- What is the absolute risk reduction in all cardiovascular outcomes in the drug group vs the placebo group? Explain this in words.
ARR (absolute risk reduction) = ARC – ART
Where ARC = the AR of events in the placebo (control) group
ART = the AR of events in the pravastatin group
ARR (absolute risk reduction)=0.20-0.18=0.20
In this study, the ARR is 20%. This means that, if 100 old age people were treated with pravastatin, 20 would be prevented from developing bad cardiovascular outcomes.
- What is the relative risk reduction in all cardiovascular outcomes in the drug group vs the placebo group? Explain this in words.
RR (relative risk) = ART / ARC
RR (relative risk) = 0.18/0.20=0.9
As the RR is less than 1, the risk of a bad cardiovascular outcome decreases, indicating that the treatment with pravastatin is beneficial.
- What is the number needed to treat? Explain this in words.
NNT (number needed to treat) = 1 / ARR
NNT (number needed to treat) =1/0.2=5
This means, 5 patients need to be treated with pravastatin instead of placebo to see 1 extra responder.
- What effect does the differential lack of compliance have? What is the direction of this bias? How does this affect internal validity?
The lack of compliance in the pravastatin group was more comparison to placebo group A differential in lack of compliance was observed as 2.7% between the two groups which is three people in every one hundred people. This bias does not favour the pravastatin group. Internal validity is affected as lack of compliance may be due selection of participants (though the study was randomized), data collection, or educating participants on drug regimen .
References:
- Szumilas, M. (2010). Explaining Odds Ratios. Journal of the Canadian Academy of Child and Adolescent Psychiatry, 19(3), 227–229.
- Spitalnic, S. (2005). Assessment I: Relative Risk and Absolute Risk Reduction. Hospital physician,43-46.
- Okwuokenye M, Zhang A, Pace A, Peace KE (2017). Number Needed to Treat in Multiple Sclerosis Clinical Trials. Neurol Ther.
- Lee, J. K., Grace, K. A., Foster, T. G., Crawley, M. J., Erowele, G. I., Sun, H. J., … Taylor, A. J. (2007). How should we measure medication adherence in clinical trials and practice? Therapeutics and Clinical Risk Management, 3(4), 685–690.
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