Read more. Subscribe Contact us. How to calculate risk. RRR is usually constant across a range of absolute risks. If a person's AR of stroke, estimated from his age and other risk factors, is 0. In a person with an AR of stroke of only 0. Because it is a ratio and expresses how many times more probable the outcome is in the exposed group, the simplest solution is to incorporate the words "times the risk" or "times as high as" in your interpretation.
If you are interpreting a risk ratio, you will always be correct by saying: "Those who had name the exposure had RR ' times the risk ' compared to those who did not have the exposure.
A cohort study examined the association between smoking and lung cancer after following smokers and non-smokers for 15 years. Which of the following would be the best interpretation of this risk ratio? There were 17 more cases of lung cancer in the smokers. Smokers had 17 times more risk of lung cancer than non-smokers. Smokers had 17 times the risk of lung cancer compared to non-smokers. When subjects who took both vitamins were compared to those who took not vitamins at all, the risk ratio was found to be 0.
Which of the following is a correct interpretation of this finding? The risk difference in this study is 70 per vitamin users over ten years. The risk difference in this study is 0. Question: Does one need to specify the time units for a risk ratio? Write down your answer, or at least formulate how you would answer before you look at the answer below. Consider an example from The Nurses' Health Study.
This prospective cohort study was used to investigate the effects of hormone replacement therapy HRT on coronary artery disease in post-menopausal women. The investigators calculated the incidence rate of coronary artery disease in post-menopausal women who had been taking HRT and compared it to the incidence rate in post-menopausal women who had not taken HRT.
The findings are summarized in this table:. Disease-free Follow-up. Interpretation: Women who used postmenopausal hormones had 0. Rate ratios are often interpreted as if they were risk ratios, e. A cohort study is conducted to determine whether smoking is associated with an increased risk of bronchitis in adults over the age of The findings are as follows:.
What is the rate ratio? All Rights Reserved. Date last modified: March 19, Wayne W. Measures of Association. Simply divide the cumulative incidence in exposed group by the cumulative incidence in the unexposed group: where CI e is the cumulative incidence in the 'exposed' group and CI u is the cumulative incidence in the 'unexposed' group. Had Incidental Appendectomy? Interpretation: In this study patients who underwent incidental appendectomy had 4.
An appropriate interpretation of this would be: Those who take low dose aspirin regularly have 0. In general: If the risk ratio is 1 or close to 1 , it suggests no difference or little difference in risk incidence in each group is the same. Percent Relative Effect An alternative way to look at and interpret these comparisons would be to compute the percent relative effect the percent change in the exposed group.
The risk of myocardial infarction among subjects taking aspirin was 0. Subjects who underwent incidental appendectomy had 4. Subjects who underwent incidental appendectomy were 4. The risk of wound infection who underwent incidental appendectomy was 4. Key Concept How to Interpret Risk Ratios: Since the relative risk is a simple ratio, errors tend to occur when the terms "more" or "less" are used.
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