Relative Risk utilizes the probability of an event occurring in one group compared to the probability of an event occurring in the other group. Relative Risk is considered a descriptive statistic, not an inferential statistic; as it does not determine statistical significance. Once we know the exposure and disease status of a research population, we can fill in their corresponding numbers in the following table. Conversely, in the aspirin study it is not correct to say that those on aspirin had 0.57 times less risk (wrong). (4.2 - 1) x 100 = 320% increase in risk. Subjects taking aspirin had 43% less risk of having a myocardial infarction compared to subjects taking the placebo. Is it those who didn't take any aspirin, those who took low-dose aspirin but used it irregularly, those who took high dose aspirin, those who took acetaminophen...? It requires the examination of two dichotomous variables, where one variable measures the event (occurred vs. not occurred) and the other variable measures the groups (group 1 vs. group 2). (Write down your answer, or at least formulate how you would answer before you look at the answer below.). (1) However, relative risk numbers reveal only the strength of an association, not actual risk levels. Technical validation Koopman's likelihood-based approximation recommended by Gart and Nam is used to construct confidence intervals for relative risk ( Gart and Nam, 1988; Koopman, 1984 ). A subject treated with AZT has 57% the chance of disease progression as a subject treated with placebo. So no evidence that drinking wine can either protect against or increase the risk of heart disease As a reminder, a risk ratio is simply a ratio of two probabilities. Question: Does one need to specify the time units for a risk ratio? Pitfalls: Note that in the interpretation of RR both the appendectomy study (in which the RR > 1), and the aspirin trial (in which RR < 1) used the expression "times the risk." RR is easy to compute and interpret and is included in standard statistical software. When RR > 1. In some sense the relative risk is a more intuitive measure of effect size. power or log utility) and some asset with a fixed "attractiveness" (essentially sharpe ratio, more on … The relative risk is 16%/28% = 0.57. This can be used to express the risk of a state, behavior or strategy as compared to a baseline risk. Subjects who underwent incidental appendectomy had 4.2 times, The rate in those using hormones was 30 / 54,308.7 = 55.2 per 100,000 person-years. Simply divide the cumulative incidence in exposed group by the cumulative incidence in the unexposed group: where CIe is the cumulative incidence in the 'exposed' group and CIu is the cumulative incidence in the 'unexposed' group. Common terms to describe these ratios are, Frequently, the term "relative risk" is used to encompass all of these. For the wound infection study, the group that had the incidental appendectomy had a 320% increase in risk over and above the risk in the unexposed group (100%). There were 17 more cases of lung cancer in the smokers. Or "The risk of (name the disease) among those who (name the exposure) was RR 'times as high as' the risk of (name the disease) among those who did not (name the exposure).". The RR is estimated as the absolute risk with the risk variable divided by the absolute risk in the control group. The rate in those NOT using hormones was 60 / 51,477.5 = 116.6 per 100,000 person-years. Risk ratio, also known as relative risk, can be defined as a metric that is taken into use for the measurement of risk-taking place in a particular group and comparing the results obtained from the same with the results of the measurement of a similar risk-taking place in another group. Relative Risk values are greater than or equal to zero. It should be clear that the hazard ratio is a relative measure of effect and tells us nothing about absolute risk. A cohort study examined the association between smoking and lung cancer after following 400 smokers and 600 non-smokers for 15 years. An appropriate interpretation of this would be: Those who take low dose aspirin regularly have 0.58 times the risk of myocardial infarction compared to those who do not take aspirin. b. For the study examining wound infections after incidental appendectomy, the risk of wound infection in each exposure group is estimated from the cumulative incidence. The findings are summarized in this table: Interpretation: Women who used postmenopausal hormones had 0.47 times the rate of coronary artery disease compared to women who did not use postmenopausal hormones. Armitage P, Berry G, Matthews JNS (2002) Statistical methods in medical research. The basic difference is that the odds ratio is a ratio of two odds (yep, it’s that obvious) whereas the relative risk is a ratio of two probabilities. Consider an example from The Nurses' Health Study. The relative risk (or risk ratio) is an intuitive way to compare the risks for the two groups. b. Smokers had 17% more lung cancers compared to non-smokers. Relative Risk Concept. The group assigned to take aspirin had an incidence of 1.26%, while the placebo (unexposed) group had an incidence of about 2.17%. https://patient.info/news-and-features/calculating-absolute-risk-and-relative-risk Literature. Let’s look at an example. (1 - 0.57) x 100 = 43% decrease in risk. the ratio of median times (median ratio) at which treatment and control group participants are at some endpoint. [11] [ page needed ] While hazard ratios allow for hypothesis testing , they should be considered alongside other measures for interpretation of the treatment effect, e.g. The relative risk or risk ratio is the ratio of the probability of an outcome in an exposed group to the probability of an outcome in an unexposed group. When RR < 1, % decrease = (1 - RR) x 100, e.g. So, the risk ratio is 5.34/1.27 or 4.2. In all cases, statistical significance is assumed if the 95% confidence interval (CI) around the relative risk does not include 1.0. For example, if we simply said, "Those who take low dose aspirin regularly have 0.58 times the risk of myocardial infarction", the question is "compared to what?" Their interpretation is similar and straightforward; a relative risk of 1.0 indicates that the risk is the same in the exposed and unexposed groups. In fact, they had 43% less risk. Which of the following is a correct interpretation of this finding? The findings are as follows: - The frequency of bronchitis in the smokers is 27 per 1,000 person-years. The relative risk (RR) of an event is the likelihood of its occurrence after exposure to a risk variable as compared with the likelihood of its occurrence in a control or reference group. (The relative risk is also called the risk ratio). d. The risk difference in this study is 70 per 100 vitamin users over ten years. RR and OR are commonly used measures of association in observational studies. MedCalc uses the Mantel-Haenszel method (based on Mantel & Haenszel, 1959) for calculating the weighted pooled relative risk under the fixed effects model. (The risk ratio is also called relative risk.) It is also possible for the risk ratio to be less than 1; this would suggest that the exposure being considered is associated with a reduction in risk. During these sessions, students can ask questions about research design, population and sampling, instrumentation, data collection, operationalizing variables, building research questions, planning data analysis, calculating sample size, study limitations, and validity. 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 is conducted to determine whether smoking is associated with an increased risk of bronchitis in adults over the age of 40. Since the relative risk is a simple ratio, errors tend to occur when the terms "more" or "less" are used. The incidence of coronary artery disease in those who take vitamins C & E daily is 0.70 (or 70%). The relative risk of a response to the mailing is the ratio of the probability that a newspaper subscriber responds, to the probability that a nonsubscriber responds. Note that the "exposure" of interest was low-dose aspirin, and the aspirin group is summarized in the top row. Subjects who underwent incidental appendectomy had 4.2, Subjects who underwent incidental appendectomy were 4.2. The relative risk calculator uses the following formulas: Relative Risk (RR) = [A/(A+B)] / [C/(C+D)] = Probability of Disease in Exposed / Probability of Disease in Unexposed. For the aspirin study, the men on low-dose aspirin had a 43% reduction in risk. At the conclusion of the study the investigators found a risk ratio = 17. In this study patients who underwent incidental appendectomy had 4.2 times the risk of post-operative wound infection compared to patients who did not undergo incidental appendectomy. The study population consisted of over 22,000 male physicians who were randomly assigned to either low-dose aspirin or a placebo (an identical looking pill that was inert). The table below shows how the risk ratio was calculated in the study examining the risk of wound infections when an incidental appendectomy was done during a staging laparotomy for Hodgkin disease. London: Chapman and Hall. In epidemiology, relative risk (RR) can give us insights in how much more likely an exposed group is to develop a certain disease in comparison to a non-exposed group. % increase = (RR - 1) x 100, e.g. It requires the examination of two dichotomous variables, where one variable measures the event (occurred vs. not occurred) and the other variable measures the groups (group 1 vs. group 2). Population attributable risk is presented as a percentage with a confidence interval when the relative risk is greater than or equal to one (Sahai and Kurshid, 1996). e. 17% of the lung cancers in smokers were due to smoking. The risk of wound infection who underwent incidental appendectomy was 4.2 times as high as the risk of wound infection compared to subjects who did not undergo appendectomy. 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. For example, if 20% of patients die with treatment A, and 15% die with treatment B, the relative risk reduction is 25%. Consider an agent with constant relative risk aversion (i.e. 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. In 1982 The Physicians' Health Study (a randomized clinical trial) was begun in order to test whether low-dose aspirin was beneficial in reducing myocardial infarctions (heart attacks). What is the rate ratio? For a short overview of meta-analysis in MedCalc, see Meta-analysis: introduction. Odds ratio vs relative risk. Next the heterogeneity statistic is incorporated to calculate the summary relative risk under the random effects model (DerSimonian & Laird, 1986). Interpretation: If Relative Risk = 1, there is no association; If Relative Risk < 1, the association is negative; If Relative Risk > 1, the association is positive The relative risk reduction is derived from the relative risk by subtracting it from one, which is the same as the ratio between the ARR and the risk in the control group. The calculated RR was reported to be 0.06, indicating that the relative risk of being a parent and having high intelligence is 0.06 times that of people who are parents and have low intelligence. Call us at 727-442-4290 (M-F 9am-5pm ET). A predictor variable with a risk ratio of less than one is often labeled a “protective factor” (at least in Epidemiology). 4 th ed. The cumulative incidence is an estimate of risk. A study is done to examine whether there is an association between the daily use of vitamins C & E and risk of coronary artery disease (heart attacks) over a 10 year period. Relative risk is a statistical term used to describe the chances of a certain event occurring among one group versus another. d. Smokers had 17 times the risk of lung cancer compared to non-smokers. Relative risk v.s. The risk of myocardial infarction among subjects taking aspirin was 0.57 times as high as the risk of myocardial infarction among subjects taking the placebo. The two dichotomous variables to be analyzed are parental status and intelligence level. risk is a ratio of event probabilities. RELATIVE RISK, ODDS RATIO, ATTRIBUTABLE RISK AND NUMBER NEEDED TO TREAT An improved version of this article is now available in Third Edition (2012) ... Risk could be 1 in 1000 or 0.05 or 0.20 but can not exceed one. Relative risk is the number that tells you how much something you do, such as maintaining a healthy weight, can change your risk compared to your risk if you're very overweight. Together with risk difference and odds ratio, relative risk measures the association between the exposure and the outcome. If you were told that your relative risk for multiple sclerosis was 10 - ie, you had a 10 fold increased risk … Consider a study where the goal is to assess the RR between parental status (a parent vs. not a parent) and intelligence level (low intelligence vs. high intelligence). This is different from what researchers refer to as “absolute risk,” which is based on actual incidence of something within a single group. 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The word “risk” is not always appropriate. To be precise, it is not correct to say that those who had an incidental appendectomy had 4.2 times more risk (wrong) or 4.2 times greater risk (wrong). How to interpret the relative risk? return to top | previous page | next page, Content ©2018. a. c. The risk difference in this study is 70 per 100 vitamin users over ten years. e. The risk difference in this study is 0.70 per 100 vitamin users over ten years. Statistics Solutions can assist with your quantitative analysis by assisting you to develop your methodology and results chapters. When subjects who took both vitamins were compared to those who took not vitamins at all, the risk ratio was found to be 0.70. In fact, those with the incidental appendectomy had a 320% increase in risk. Relative risk can be expressed as a percentage decrease or a percentage increase. Blackwell Science. • The relative risk reductionis the difference in event rates between two groups, expressed as a proportion of the event rate in the untreated group. In essence, we regard the unexposed group as having 100% of the risk and express the exposed group relative to that. c. Smokers had 17 times more risk of lung cancer than non-smokers. Don't see the date/time you want? Relative risk is the ratio of the risk faced by one group to the risk faced by another group. Risk ratios are a bit trickier to interpret when they are less than one. Which of the following would be the best interpretation of this risk ratio? It is commonly used in epidemiology and evidence-based medicine, … Date last modified: March 19, 2018. Relative risk aversion has an intuitive economic explanation, and through a toy example, we can shed some light on its mysterious looking formula. a. Some of the data is summarized in the 2x2 table shown below. A risk ratio < 1 suggests a reduced risk in the exposed group. Menu location: Analysis_Meta-Analysis_Relative Risk. Likewise, Note also that the unexposed (comparison, reference) group must be specified. A value >1 suggests increase risk, while a value <1 suggest reduction of risk. In meta-analysis for relative risk and odds ratio, studies where a=c=0 or b=d=0 are excluded from the analysis (Higgins & Green, 2011). (Rate ratios are often interpreted as if they were risk ratios, e.g., post-menopausal women using HRT had 0.47 times the risk of CAD compared to women not using HRT, but it is more precise to refer to the ratio of rates rather than risk.). Especially while coefficients in logistic regression are directly interpreted as (adjusted) odds ratio, they are unwittingly translated as (adjusted) relative risks in many public health studies. A relative risk of 3.0 means the rate is three times as high (200 percent more) … and so forth. 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