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. Let’s look at an example. This can be used to express the risk of a state, behavior or strategy as compared to a baseline risk. The word “risk” is not always appropriate. A relative risk of 1.0 indicates no difference between comparison groups. 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. Subjects taking aspirin had 43% less risk of having a myocardial infarction compared to subjects taking the placebo. Since the relative risk is a simple ratio, errors tend to occur when the terms "more" or "less" are used. The following is the interpretation of the multinomial logistic regression in terms of relative risk ratios and can be obtained by mlogit, rrr after 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). A subject treated with AZT has 57% the chance of disease progression as a subject treated with placebo. 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 . Consider an example from The Nurses' Health Study. The findings are as follows: - The frequency of bronchitis in the smokers is 27 per 1,000 person-years. 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 rate in those NOT using hormones was 60 / 51,477.5 = 116.6 per 100,000 person-years. a. A predictor variable with a risk ratio of less than one is often labeled a “protective factor” (at least in Epidemiology). 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. Next the heterogeneity statistic is incorporated to calculate the summary relative risk under the random effects model (DerSimonian & Laird, 1986). e. The risk difference in this study is 0.70 per 100 vitamin users over ten years. Some of the data is summarized in the 2x2 table shown below. Calculation. Wayne W. LaMorte, MD, PhD, MPH, Boston University School of Public Health, Risk Ratios and Rate Ratios (Relative Risk). 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. Statistics Solutions can assist with your quantitative analysis by assisting you to develop your methodology and results chapters. Likewise, 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). The relative risk (or risk ratio) is an intuitive way to compare the risks for the two groups. (The relative risk is also called the risk ratio). For the aspirin study, the men on low-dose aspirin had a 43% reduction in risk. 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 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. https://patient.info/news-and-features/calculating-absolute-risk-and-relative-risk The incidence of coronary artery disease in those who take vitamins C & E daily is 0.70 (or 70%). 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. There were 17 more cases of lung cancer in the smokers. 4 th ed. If you were told that your relative risk for multiple sclerosis was 10 - ie, you had a 10 fold increased risk … d. Smokers had 17 times the risk of lung cancer compared to non-smokers. 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. 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. Menu location: Analysis_Meta-Analysis_Relative Risk. It should be clear that the hazard ratio is a relative measure of effect and tells us nothing about absolute risk. (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.). Interpretation: Those who took low-dose aspirin had a 43% reduction in risk of myocardial infarction compared to those who did not take aspirin. Cohort studies of dichotomous outcomes (e.g. RR is easy to compute and interpret and is included in standard statistical software. 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).". Thus, the estimate of the relative risk is simply 13.7%/8.2% = 1.668. 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...? Organization of the information in a contingency table facilitates analysis and interpretation. Relative Risk (RR) is often used when the study involves comparing the likelihood, or chance, of an event occurring between two groups. 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. 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. 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. In essence, we regard the unexposed group as having 100% of the risk and express the exposed group relative to that. At the conclusion of the study the investigators found a risk ratio = 17. 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. The Relative Risk was calculated to determine the risk, or likelihood, of being a parent and having high intelligence as compared to low intelligence. For example, if 20% of patients die with treatment A, and 15% die with treatment B, the relative risk reduction is 25%. 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). A relative risk of 3.0 means the rate is three times as high (200 percent more) … and so forth. Call us at 727-442-4290 (M-F 9am-5pm ET). - The frequency of bronchitis in the non-smokers is 3 per 1,000 person-years. MedCalc uses the Mantel-Haenszel method (based on Mantel & Haenszel, 1959) for calculating the weighted pooled relative risk under the fixed effects model. Relative risk can be expressed as a percentage decrease or a percentage increase. (The risk ratio is also called relative risk.) 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?" For example. So no evidence that drinking wine can either protect against or increase the risk of heart disease These relative measures give an indication of the "strength of association.". Relative risk and odds ratio are often confused or misinterpreted. This prospective cohort study was used to investigate the effects of hormone replacement therapy (HRT) on coronary artery disease in post-menopausal women. • The relative risk reductionis the difference in event rates between two groups, expressed as a proportion of the event rate in the untreated group. Think of the relative risk as being simply the ratio of proportions. 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%). The group assigned to take aspirin had an incidence of 1.26%, while the placebo (unexposed) group had an incidence of about 2.17%. 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. 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). Relative Risk is considered a descriptive statistic, not an inferential statistic; as it does not determine statistical significance. Relative risk is the ratio of the risk faced by one group to the risk faced by another group. 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. 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. For the study examining wound infections after incidental appendectomy, the risk of wound infection in each exposure group is estimated from the cumulative incidence. It is commonly used in epidemiology and evidence-based medicine, … 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 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). 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. 9.2.2.2 Measures of relative effect: the risk ratio and odds ratio. 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. 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. 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. What is the rate ratio? A cohort study is conducted to determine whether smoking is associated with an increased risk of bronchitis in adults over the age of 40. A relative risk of 0.5 means that your risk is 1/2 that of average or a 50% lower risk. So it’s important to keep them separate and to be precise in the language you use. 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 In fact, they had 43% less risk. 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. 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(Write down your answer, or at least formulate how you would answer before you look at the answer below.). 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. dead or alive) can by represented by arranging the observed counts into fourfold (2 by 2) tables. % increase = (RR - 1) x 100, e.g. Literature. return to top | previous page | next page, Content ©2018. So, the risk ratio is 5.34/1.27 or 4.2. For a short overview of meta-analysis in MedCalc, see Meta-analysis: introduction. Question: Does one need to specify the time units for a risk ratio? Which of the following would be the best interpretation of this risk ratio? Measures of disease frequency can be compared by calculating their ratio. The RR is estimated as the absolute risk with the risk variable divided by the absolute risk in the control group. 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. Common terms to describe these ratios are, Frequently, the term "relative risk" is used to encompass all of these. 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). Conversely, in the aspirin study it is not correct to say that those on aspirin had 0.57 times less risk (wrong). Together with risk difference and odds ratio, relative risk measures the association between the exposure and the outcome. 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. It is a decimal number although often expressed as percentage. All Rights Reserved. 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. RR and OR are commonly used measures of association in observational studies. Risk ratios are a bit trickier to interpret when they are less than one. The cumulative incidence is an estimate of risk. The relative risk and odds ratio of 1 suggests that there is no difference between two groups. 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). 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. 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 risk ratio < 1 suggests a reduced risk in the exposed group. b. Smokers had 17% more lung cancers compared to non-smokers. Subjects who underwent incidental appendectomy had 4.2, Subjects who underwent incidental appendectomy were 4.2. odds ratio. (1 - 0.57) x 100 = 43% decrease in risk. Note that the "exposure" of interest was low-dose aspirin, and the aspirin group is summarized in the top row. 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). A risk ratio > 1 suggests an increased risk of that outcome in the exposed group. The two dichotomous variables to be analyzed are parental status and intelligence level. Once we know the exposure and disease status of a research population, we can fill in their corresponding numbers in the following table. In fact, those with the incidental appendectomy had a 320% increase in risk. London: Chapman and Hall. Relative risk v.s. Which of the following is a correct interpretation of this finding? Note also that the unexposed (comparison, reference) group must be specified. d. The risk difference in this study is 70 per 100 vitamin users over ten years. 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. 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%. Interpretation: Those who had the incidental appendectomy had a 320% increase in risk of getting a post-operative wound infection. Relative risk is a statistical term used to describe the chances of a certain event occurring among one group versus another. Those who take vitamins C & E daily have 0.7 times the risk of heart attack compared to those who do not take vitamins. Measures of relative effect express the outcome in one group relative to that in the other. 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." 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). In all cases, statistical significance is assumed if the 95% confidence interval (CI) around the relative risk does not include 1.0. 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. a. c. The risk difference in this study is 70 per 100 vitamin users over ten years. 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. When RR < 1, % decrease = (1 - RR) x 100, e.g. (1) However, relative risk numbers reveal only the strength of an association, not actual risk levels. Blackwell Science. A value >1 suggests increase risk, while a value <1 suggest reduction of risk. 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. b. Date last modified: March 19, 2018. As a reminder, a risk ratio is simply a ratio of two probabilities. 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)." The relative risk is 16%/28% = 0.57. In some sense the relative risk is a more intuitive measure of effect size. 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. Odds ratio vs relative 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). Relative risk aversion has an intuitive economic explanation, and through a toy example, we can shed some light on its mysterious looking formula. The cumulative incidence in the aspirin group was divided by the cumulative incidence in the placebo group, and RR= 0.58. The relative risk (or risk ratio) is an intuitive way to compare the risks for the two groups. We can similarly calculate the cumulative incidence in the patients who did not have an incidental appendectomy, which was 1 divided by 79 or 1.27%. 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 relative risk is a ratio and does not follow a normal distribution, regardless of the sample sizes in the comparison groups. How to interpret the relative risk? They followed these physicians for about five years. Consider an agent with constant relative risk aversion (i.e. 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. Meta-analysis may be used to investigate the combination or interaction of a group of independent studies, for example a series of fourfold tables from similar studies conducted at different centres. the ratio of median times (median ratio) at which treatment and control group participants are at some endpoint. Don't see the date/time you want? 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. Relative Risk Concept. (4.2 - 1) x 100 = 320% increase in risk. The relative risk (also called the risk ratio or prevalence ratio or relative prevalence) is Easy to interpret and explain Often the quantity of interest (although additive risk should also be considered) Estimable via relative risk regression using standard statistical software Altman DG (1991) Practical statistics for medical research. 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. risk is a ratio of event probabilities. Armitage P, Berry G, Matthews JNS (2002) Statistical methods in medical research. 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. 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