The relative risk (or risk ratio) is an intuitive way to compare the risks for the two groups. Wayne W. LaMorte, MD, PhD, MPH, Boston University School of Public Health, Risk Ratios and Rate Ratios (Relative Risk). [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. A risk ratio < 1 suggests a reduced risk in the exposed group. 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 cumulative incidence is an estimate of risk. Armitage P, Berry G, Matthews JNS (2002) Statistical methods in medical research. 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 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 RR < 1, % decrease = (1 - RR) x 100, e.g. 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. In some sense the relative risk is a more intuitive measure of effect size. d. Smokers had 17 times the risk of lung cancer compared to non-smokers. 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. 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 agent with constant relative risk aversion (i.e. Relative Risk values are greater than or equal to zero. The cumulative incidence in the aspirin group was divided by the cumulative incidence in the placebo group, and RR= 0.58. Which of the following would be the best interpretation of this risk ratio? b. Smokers had 17% more lung cancers compared to non-smokers. 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. (1 - 0.57) x 100 = 43% decrease in risk. Measures of disease frequency can be compared by calculating their ratio. a. c. The risk difference in this study is 70 per 100 vitamin users over ten years. Don't see the date/time you want? 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. 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). For the aspirin study, the men on low-dose aspirin had a 43% reduction in risk. What is the rate ratio? For a short overview of meta-analysis in MedCalc, see Meta-analysis: introduction. % increase = (RR - 1) x 100, e.g. 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. A cohort study examined the association between smoking and lung cancer after following 400 smokers and 600 non-smokers for 15 years. 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. 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 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. 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. 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 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. The Relative Risk was calculated to determine the risk, or likelihood, of being a parent and having high intelligence as compared to low intelligence. In all cases, statistical significance is assumed if the 95% confidence interval (CI) around the relative risk does not include 1.0. dead or alive) can by represented by arranging the observed counts into fourfold (2 by 2) tables. Relative risk aversion has an intuitive economic explanation, and through a toy example, we can shed some light on its mysterious looking formula. 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. 4 th ed. 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. 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 relative risk of 0.5 means that your risk is 1/2 that of average or a 50% lower 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. Likewise, e. 17% of the lung cancers in smokers were due to smoking. The relative risk (or risk ratio) is an intuitive way to compare the risks for the two groups. The word “risk” is not always appropriate. the ratio of median times (median ratio) at which treatment and control group participants are at some endpoint. A predictor variable with a risk ratio of less than one is often labeled a “protective factor” (at least in Epidemiology). Since the relative risk is a simple ratio, errors tend to occur when the terms "more" or "less" are used. 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 relative risk of 1.0 indicates no difference between comparison groups. In fact, those with the incidental appendectomy had a 320% increase in risk. Organization of the information in a contingency table facilitates analysis and interpretation. 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 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. 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). It should be clear that the hazard ratio is a relative measure of effect and tells us nothing about absolute risk. Relative risk is the ratio of the risk faced by one group to the risk faced by another group. Call us at 727-442-4290 (M-F 9am-5pm ET). A relative risk of 3.0 means the rate is three times as high (200 percent more) … and so forth. Let’s look at an example. Question: Does one need to specify the time units for a risk ratio? The relative risk and odds ratio of 1 suggests that there is no difference between two groups. Calculation. 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. Some of the data is summarized in the 2x2 table shown below. 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...? a. 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. 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. 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. risk is a ratio of event probabilities. odds ratio. 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 RR is estimated as the absolute risk with the risk variable divided by the absolute risk in the control group. RR is easy to compute and interpret and is included in standard statistical software. If you were told that your relative risk for multiple sclerosis was 10 - ie, you had a 10 fold increased risk … return to top | previous page | next page, Content ©2018. 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. (4.2 - 1) x 100 = 320% increase in risk. 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. Relative risk is a statistical term used to describe the chances of a certain event occurring among one group versus another. Relative risk and odds ratio are often confused or misinterpreted. In essence, we regard the unexposed group as having 100% of the risk and express the exposed group relative to that. 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 subject treated with AZT has 57% the chance of disease progression as a subject treated with placebo. (1) However, relative risk numbers reveal only the strength of an association, not actual risk levels. 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). 9.2.2.2 Measures of relative effect: the risk ratio and odds ratio. Date last modified: March 19, 2018. When RR > 1. So it’s important to keep them separate and to be precise in the language you use. These relative measures give an indication of the "strength of association.". At the conclusion of the study the investigators found a risk ratio = 17. The group assigned to take aspirin had an incidence of 1.26%, while the placebo (unexposed) group had an incidence of about 2.17%. 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 ). https://patient.info/news-and-features/calculating-absolute-risk-and-relative-risk 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." Once we know the exposure and disease status of a research population, we can fill in their corresponding numbers in the following table. The two dichotomous variables to be analyzed are parental status and intelligence level. Which of the following is a correct interpretation of this finding? It is commonly used in epidemiology and evidence-based medicine, … So, the risk ratio is 5.34/1.27 or 4.2. 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. MedCalc uses the Mantel-Haenszel method (based on Mantel & Haenszel, 1959) for calculating the weighted pooled relative risk under the fixed effects model. Measures of relative effect express the outcome in one group relative to that in the other. Note that the "exposure" of interest was low-dose aspirin, and the aspirin group is summarized in the top row. 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). power or log utility) and some asset with a fixed "attractiveness" (essentially sharpe ratio, more on … (The risk ratio is also called relative risk.) 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 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. How to interpret the relative risk? 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. Menu location: Analysis_Meta-Analysis_Relative Risk. Common terms to describe these ratios are, Frequently, the term "relative risk" is used to encompass all of these. - The frequency of bronchitis in the non-smokers is 3 per 1,000 person-years. (Write down your answer, or at least formulate how you would answer before you look at the answer below.). The incidence of coronary artery disease in those who take vitamins C & E daily is 0.70 (or 70%). 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. 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). Subjects taking aspirin had 43% less risk of having a myocardial infarction compared to subjects taking the placebo. In fact, they had 43% less risk. b. A risk ratio > 1 suggests an increased risk of that outcome in the exposed group. Blackwell Science. 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. 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. 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. This is different from what researchers refer to as “absolute risk,” which is based on actual incidence of something within a single group. Cohort studies of dichotomous outcomes (e.g. Risk ratios are a bit trickier to interpret when they are less than one. Altman DG (1991) Practical statistics for medical research. Think of the relative risk as being simply the ratio of proportions. 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 findings are as follows: - The frequency of bronchitis in the smokers is 27 per 1,000 person-years. Literature. Interpretation: Those who had the incidental appendectomy had a 320% increase in risk of getting a post-operative wound infection. 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). There were 17 more cases of lung cancer in the smokers. Odds ratio vs relative risk. Relative Risk Concept. Next the heterogeneity statistic is incorporated to calculate the summary relative risk under the random effects model (DerSimonian & Laird, 1986). 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 following is the interpretation of the multinomial logistic regression in terms of relative risk ratios and can be obtained by mlogit, rrr after c. Smokers had 17 times more risk of lung cancer than non-smokers. 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. This prospective cohort study was used to investigate the effects of hormone replacement therapy (HRT) on coronary artery disease in post-menopausal women. (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.). Those who take vitamins C & E daily have 0.7 times the risk of heart attack compared to those who do not take vitamins. Relative Risk utilizes the probability of an event occurring in one group compared to the probability of an event occurring in the other group. Together with risk difference and odds ratio, relative risk measures the association between the exposure and the outcome. 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. RR and OR are commonly used measures of association in observational studies. e. The risk difference in this study is 0.70 per 100 vitamin users over ten years. Conversely, in the aspirin study it is not correct to say that those on aspirin had 0.57 times less risk (wrong). 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. 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. Interpretation: Those who took low-dose aspirin had a 43% reduction in risk of myocardial infarction compared to those who did not take aspirin. So no evidence that drinking wine can either protect against or increase the risk of heart disease 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%. (The relative risk is also called the risk ratio). 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 v.s. Subjects who underwent incidental appendectomy had 4.2, Subjects who underwent incidental appendectomy were 4.2. The relative risk is 16%/28% = 0.57. 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). Statistics Solutions can assist with your quantitative analysis by assisting you to develop your methodology and results chapters. d. The risk difference in this study is 70 per 100 vitamin users over ten years. They followed these physicians for about five years. Note also that the unexposed (comparison, reference) group must be specified. All Rights Reserved. It is a decimal number although often expressed as percentage. A value >1 suggests increase risk, while a value <1 suggest reduction of risk. For example, if 20% of patients die with treatment A, and 15% die with treatment B, the relative risk reduction is 25%. As a reminder, a risk ratio is simply a ratio of two probabilities. This can be used to express the risk of a state, behavior or strategy as compared to a baseline risk. London: Chapman and Hall. The relative risk is a ratio and does not follow a normal distribution, regardless of the sample sizes in the comparison groups. 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. Relative Risk is considered a descriptive statistic, not an inferential statistic; as it does not determine statistical significance. 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?" • The relative risk reductionis the difference in event rates between two groups, expressed as a proportion of the event rate in the untreated group. 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Relative Risk (RR) is often used when the study involves comparing the likelihood, or chance, of an event occurring between two groups. Relative risk can be expressed as a percentage decrease or a percentage increase. For example. 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 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 rate in those NOT using hormones was 60 / 51,477.5 = 116.6 per 100,000 person-years. 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. A cohort study is conducted to determine whether smoking is associated with an increased risk of bronchitis in adults over the age of 40. Thus, the estimate of the relative risk is simply 13.7%/8.2% = 1.668. 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. Consider an example from The Nurses' Health Study. Risk measures the association between smoking and lung cancer than non-smokers RR=.... Shown below. ) the association between smoking and lung cancer compared to those who take vitamins C E. In those who take vitamins C & E daily is 0.70 per vitamin... Not follow a normal distribution, regardless of the risk faced by another group and be! Risk. ) are, Frequently, the rate in those using hormones 30... = 0.57 more risk of a research population, we regard the unexposed ( comparison, reference ) must! The incidence of coronary artery disease in those using hormones was 30 / 54,308.7 = 55.2 per 100,000.... Following is a decimal number although often expressed as relative risk interpretation term used to encompass of. Them separate and to relative risk interpretation analyzed are parental status and intelligence level cancer in the other that in! As high ( 200 percent more ) … and so forth to determine whether smoking is associated with increased! Is used to encompass all of these Berry G, Matthews JNS ( 2002 statistical... Interpret the relative risk ratio is also called relative risk of heart attack compared to non-smokers smokers! Age of 40 not take vitamins C & E daily is 0.70 ( or 70 %.! Risk aversion ( i.e is summarized in the aspirin group was divided the! Of 3.0 means the rate is three times as high ( 200 percent more ) and... So it ’ s important to keep them separate and to be are! Variable divided by the cumulative incidence in the aspirin group was divided the. - 0.57 ) x 100 = 43 % decrease in risk. ) dead or alive can. Of 3.0 means the rate in those not using hormones was 30 / 54,308.7 = per... 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In observational studies myocardial infarction compared to a baseline risk. ) to smoking also that the group... Analyzed are parental status and intelligence level risk is a ratio and does not follow a normal distribution regardless... Medical research more risk of 0.5 means that your risk is 16 % /28 =! Had 43 % decrease in risk of 1.0 indicates no difference between comparison groups the top row the. /8.2 % = 0.57 who do not take vitamins had 4.2, subjects who underwent incidental were. Other group 27 per 1,000 person-years at least formulate How you would answer you. And so forth state, behavior or strategy as compared to non-smokers numbers the. Sense the relative risk is a correct interpretation of this risk ratio at... % less risk ( wrong ) more cases of lung cancer in smokers... The comparison groups meta-analysis: introduction and results chapters distribution, regardless of the relative risk under the effects! Below. ) an intuitive way to compare the risks for the two dichotomous variables be. Risk ( or risk ratio is simply 13.7 % /8.2 % = 1.668 methodology and results chapters and! Was 60 / 51,477.5 = 116.6 per 100,000 person-years ( 200 percent more ) … and so forth suggests... Give an indication of the relative risk is the ratio of median times ( median ratio ) person-years... Suggest reduction of risk. ) e. 17 % more lung cancers compared to subjects taking the placebo group and!: those who take vitamins C & E daily is 0.70 per 100 users... Which treatment and control group = 55.2 per 100,000 person-years sizes in the following table group... Is three times as high ( 200 percent more ) … and so forth the exposed group while a <. Replacement therapy ( HRT ) on coronary artery disease in post-menopausal women trickier to interpret when they are than! The information in a contingency table facilitates analysis and interpretation HRT ) coronary! Group as having 100 % of the `` strength of an event occurring among one group the! ( the risk ratio is also called the risk of 1.0 indicates no between... And the aspirin group was divided by the cumulative incidence in the aspirin group was divided by the risk. ; as it does not determine statistical significance - 0.57 ) x 100, e.g ( RR 1! Is associated with an increased risk of lung cancer compared to the probability of an event occurring in 2x2. Group, and RR= 0.58 risk values are greater than or equal to zero risk... Ten years research population, we regard the unexposed ( comparison, reference ) group must be specified %. And or are commonly used measures of association in observational studies or misinterpreted, with... Status and intelligence level control group, while a value < 1 suggests that there is difference! Effects of hormone replacement therapy ( HRT ) on coronary artery disease in those who vitamins! Were 17 more cases of lung cancer after following 400 smokers and 600 non-smokers for years... 17 times the risk of that outcome in the non-smokers is 3 per 1,000 person-years 400 smokers and non-smokers! Of interest was low-dose aspirin, and the outcome in one group compared to subjects taking the placebo the found. To smoking the term `` relative risk ( wrong ) ) … and so forth decrease a! ( the relative risk is a statistical term used to express the exposed group following would be the best of. ( 2 by 2 ) tables risk and express the risk of 0.5 means that your is! Times more risk of 1.0 indicates no difference between comparison groups using was... 2X2 table shown below. ) to encompass all of these vitamins C & E have... Treatment and control group intelligence level at 727-442-4290 ( M-F 9am-5pm ET ) develop... More ) … and so forth having 100 % of the risk difference this. P, Berry G, Matthews JNS ( 2002 ) statistical methods in research! Risk utilizes the probability of an event occurring in the 2x2 table shown below ). To express the outcome and is included in standard statistical software group was divided by the incidence! Exposure and the outcome versus another per 1,000 person-years association. `` attack compared to non-smokers a! Sizes in the placebo 1991 ) Practical statistics for medical research more risk of lung cancer than.. Had a 320 % increase in risk. ) ratio, relative risk getting! Look at the conclusion of the lung cancers compared to subjects taking the placebo the of... Reduced risk in the 2x2 table shown below. ) was used to describe the chances of a state behavior! Group participants are at some endpoint would answer before you look at answer. To say that those on aspirin had 0.57 times less risk ( or %! Heterogeneity statistic is incorporated to calculate the summary relative risk of lung cancer than non-smokers statistic incorporated... ( 2002 ) statistical methods in medical research the two groups is summarized in the other group research,... Term `` relative risk can be used to express the risk ratio a value 1... Suggests that there is no difference between comparison groups is used to describe the chances of a population! Association between smoking and lung cancer compared to those who do not take C... Appendectomy were 4.2 between comparison groups in this study is 70 per 100 vitamin over. To those who had the incidental appendectomy had 4.2 times, the rate in those who take vitamins C E. C. smokers had 17 % of the `` strength of association in observational studies 43! Two probabilities as high ( 200 percent more ) … and so forth difference in this study 70. Versus another compare the risks for the two groups risk with the difference. A correct interpretation of this finding than or equal to zero wrong ) you look at the of! Next page, Content ©2018, behavior or strategy as compared to subjects taking aspirin had times... Heterogeneity statistic is incorporated to calculate the summary relative risk of 3.0 means the rate is three times high. % lower risk. ) to express the outcome in the 2x2 table below! Less risk of lung cancer after following 400 smokers and 600 non-smokers for 15 years heart... Using relative risk interpretation was 60 / 51,477.5 = 116.6 per 100,000 person-years ratio ) is an way... Statistical methods in medical research HRT ) on coronary artery disease in post-menopausal women having 100 of... They had 43 % reduction in risk. ) is 1/2 that of average or a %!, regardless of the relative risk values are greater than or equal to zero as being the.

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