A relative risk of 1.0 indicates no difference between comparison groups. Relative risk can be expressed as a percentage decrease or a percentage increase. Think of the relative risk as being simply the ratio of proportions. the ratio of median times (median ratio) at which treatment and control group participants are at some endpoint. Relative Risk (RR) is often used when the study involves comparing the likelihood, or chance, of an event occurring between two groups. 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. 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). Since the relative risk (RR) is the ratio of 2 numbers, we can expect 1 of 3 options: RR = 1: The risk in the first group is the same as the risk in the second. A relative risk of 1.5 means you have a 50% higher risk than average; A relative risk of 10 means you have 10 times the average risk; Puttng relative risk into context will mean you will need to know the baseline risk of disease . The Relative Risk was calculated to determine the risk, or likelihood, of being a parent and having high intelligence as compared to low intelligence. 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...? odds ratio. A predictor variable with a risk ratio of less than one is often labeled a “protective factor” (at least in Epidemiology). (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.). Relative risk aversion has an intuitive economic explanation, and through a toy example, we can shed some light on its mysterious looking formula. 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). Relative Risk is very similar to Odds Ratio, however, RR is calculated by using percentages, whereas Odds Ratio is calculated by using the ratio of odds. 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. Incidental appendectomies were performed in a total of 131 patients, and seven of these developed post-operative wound infections, so the cumulative incidence was 7 divided by 131, or 5.34%. Relative risk and odds ratio are often confused or misinterpreted. Odds ratios and relative risks are interpreted in much the same way and if and are much less than and then the odds ratio will be almost the same as the relative 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). Consider an agent with constant relative risk aversion (i.e. For the aspirin study, the men on low-dose aspirin had a 43% reduction in risk. These relative measures give an indication of the "strength of association.". The relative risk is a ratio and does not follow a normal distribution, regardless of the sample sizes in the comparison groups. Call us at 727-442-4290 (M-F 9am-5pm ET). Relative risk is the ratio of the risk faced by one group to the risk faced by another group. (The risk ratio is also called relative risk.) RR is easy to compute and interpret and is included in standard statistical software. Relative Risk utilizes the probability of an event occurring in one group compared to the probability of an event occurring in the other group. Wayne W. LaMorte, MD, PhD, MPH, Boston University School of Public Health, Risk Ratios and Rate Ratios (Relative Risk). Blackwell Science. It is commonly used in epidemiology and evidence-based medicine, … A value of 1 indicates a neutral result: the chance of an event occurring for one group is the same for an event occurring for the other group. Statistics Solutions can assist with your quantitative analysis by assisting you to develop your methodology and results chapters. The findings are as follows: - The frequency of bronchitis in the smokers is 27 per 1,000 person-years. b. Smokers had 17% more lung cancers compared to non-smokers. return to top | previous page | next page, Content ©2018. Let’s look at an example. They followed these physicians for about five years. The rate in those NOT using hormones was 60 / 51,477.5 = 116.6 per 100,000 person-years. b. Cohort studies of dichotomous outcomes (e.g. For example. 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." A relative risk of 3.0 means the rate is three times as high (200 percent more) … and so forth. 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). Note that the "exposure" of interest was low-dose aspirin, and the aspirin group is summarized in the top row. d. Smokers had 17 times the risk of lung cancer compared to non-smokers. 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?" Armitage P, Berry G, Matthews JNS (2002) Statistical methods in medical research. London: Chapman and Hall. % increase = (RR - 1) x 100, e.g. The relative risk (or risk ratio) is an intuitive way to compare the risks for the two groups. c. Smokers had 17 times more risk of lung cancer than non-smokers. The word “risk” is not always appropriate. The following is the interpretation of the multinomial logistic regression in terms of relative risk ratios and can be obtained by mlogit, rrr after What is the rate ratio? A value >1 suggests increase risk, while a value <1 suggest reduction of risk. In fact, those with the incidental appendectomy had a 320% increase in risk. At the conclusion of the study the investigators found a risk ratio = 17. 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. c. The risk difference in this study is 70 per 100 vitamin users over ten years. It is a decimal number although often expressed as percentage. A subject treated with AZT has 57% the chance of disease progression as a subject treated with placebo. In fact, they had 43% less risk. Measures of disease frequency can be compared by calculating their ratio. Once we know the exposure and disease status of a research population, we can fill in their corresponding numbers in the following table. (The relative risk is also called the risk ratio). RR and OR are commonly used measures of association in observational studies. This is different from what researchers refer to as “absolute risk,” which is based on actual incidence of something within a single group. 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. Consider an example from The Nurses' Health Study. So, the risk ratio is 5.34/1.27 or 4.2. A risk ratio > 1 suggests an increased risk of that outcome in the exposed group. risk is a ratio of event probabilities. 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).". A risk ratio < 1 suggests a reduced risk in the exposed group. 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. Relative Risk is calculated by dividing the probability of an event occurring for group 1 (A) divided by the probability of an event occurring for group 2 (B). MedCalc uses the Mantel-Haenszel method (based on Mantel & Haenszel, 1959) for calculating the weighted pooled relative risk under the fixed effects model. Subjects who underwent incidental appendectomy had 4.2, Subjects who underwent incidental appendectomy were 4.2. A cohort study is conducted to determine whether smoking is associated with an increased risk of bronchitis in adults over the age of 40. Together with risk difference and odds ratio, relative risk measures the association between the exposure and the outcome. 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. Measures of relative effect express the outcome in one group relative to that in the other. After collecting data, the following was reported: 32 subjects were parents and had high intelligence, 676 subjects were not parents and had high intelligence, 26 subjects were parents and had low intelligence, and 8 subjects were not parents and had low intelligence. d. The risk difference in this study is 70 per 100 vitamin users over ten years. A relative risk of 0.5 means that your risk is 1/2 that of average or a 50% lower 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). The RR is estimated as the absolute risk with the risk variable divided by the absolute risk in the control group. 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 relative risk reductionis the difference in event rates between two groups, expressed as a proportion of the event rate in the untreated group. e. 17% of the lung cancers in smokers were due to smoking. However, a value of zero indicates that none of the cases in group 1 had the event occur while x number of cases in group 2 had the event occur; or in other words, the numerator was a zero (A = 0) and the denominator was any number greater than zero (B = x, where x > 0). 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 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). 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. https://patient.info/news-and-features/calculating-absolute-risk-and-relative-risk a. Relative risk is a statistical term used to describe the chances of a certain event occurring among one group versus another. 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. So it’s important to keep them separate and to be precise in the language you use. 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). - The frequency of bronchitis in the non-smokers is 3 per 1,000 person-years. When RR > 1. 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). Relative Risk is considered a descriptive statistic, not an inferential statistic; as it does not determine statistical significance. The relative risk and odds ratio of 1 suggests that there is no difference between two groups. 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. 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. 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). Note also that the unexposed (comparison, reference) group must be specified. 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Subjects taking aspirin had 43% less risk of having a myocardial infarction compared to subjects taking the placebo. Common terms to describe these ratios are, Frequently, the term "relative risk" is used to encompass all of these. There were 17 more cases of lung cancer in the smokers. 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. For a short overview of meta-analysis in MedCalc, see Meta-analysis: introduction. In some sense the relative risk is a more intuitive measure of effect size. 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. Those who take vitamins C & E daily have 0.7 times the risk of heart attack compared to those who do not take vitamins. 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. The incidence of coronary artery disease in those who take vitamins C & E daily is 0.70 (or 70%). This can be used to express the risk of a state, behavior or strategy as compared to a baseline risk. 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 ). Don't see the date/time you want? Relative Risk utilizes the probability of an event occurring in one group compared to the probability of an event occurring in the other group. [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. For the study examining wound infections after incidental appendectomy, the risk of wound infection in each exposure group is estimated from the cumulative incidence. For example, if 20% of patients die with treatment A, and 15% die with treatment B, the relative risk reduction is 25%. (4.2 - 1) x 100 = 320% increase in risk. 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. The parameter of interest is the relative risk or risk ratio in the population, RR=p 1 /p 2, and the point estimate is the RR obtained from our samples. Odds ratio vs relative risk. 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. Organization of the information in a contingency table facilitates analysis and interpretation. Calculation. The two dichotomous variables to be analyzed are parental status and intelligence level. The cumulative incidence in the aspirin group was divided by the cumulative incidence in the placebo group, and RR= 0.58. All Rights Reserved. Next the heterogeneity statistic is incorporated to calculate the summary relative risk under the random effects model (DerSimonian & Laird, 1986). 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. This prospective cohort study was used to investigate the effects of hormone replacement therapy (HRT) on coronary artery disease in post-menopausal women. 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). Relative Risk Concept. (Write down your answer, or at least formulate how you would answer before you look at the answer below.). Which of the following would be the best interpretation of this risk ratio? Likewise, If you were told that your relative risk for multiple sclerosis was 10 - ie, you had a 10 fold increased risk … dead or alive) can by represented by arranging the observed counts into fourfold (2 by 2) tables. (1 - 0.57) x 100 = 43% decrease in risk. The relative risk (or risk ratio) is an intuitive way to compare the risks for the two groups. 4 th ed. e. The risk difference in this study is 0.70 per 100 vitamin users over ten years. How to interpret the relative risk? Rate ratios are closely related to risk ratios, but they are computed as the ratio of the incidence rate in an exposed group divided by the incidence rate in an unexposed (or less exposed) comparison group. 9.2.2.2 Measures of relative effect: the risk ratio and odds ratio. When RR < 1, % decrease = (1 - RR) x 100, e.g. 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. In essence, we regard the unexposed group as having 100% of the risk and express the exposed group relative to that. Relative risk v.s. 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. Some of the data is summarized in the 2x2 table shown below. Risk ratios are a bit trickier to interpret when they are less than one. 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( the risk difference in this study is 0.70 ( or risk ratio = 17 meta-analysis:.. ) at which treatment and control group participants are at some endpoint is an intuitive way to compare risks! To smoking give an indication of the study the investigators found a risk ratio is also relative.