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 (The relative risk is also called the risk ratio) Let's look at an example Relative Risk/Risk Ratio Suppose you have a school that wants to test out a new tutoring programThe relative risk (RR) and the odds ratio (OR) are the two most widely used measures of association in epidemiology The direct computation of relative risks isRelative Risk, Odds, and Fisher's exact test I) Relative Risk A) Simply, relative risk is the ratio of p 1/p 2 For instance, suppose we wanted to take another look at our Seat belt safety data from Florida Safety equipment Injury in use Fatal Nonfatal Total None 1,601 165,527 167,128 Seat belt 510 412,368 412,878
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Odds ratio vs relative risk study- The odds ratio (OR) is the ratio of the odds of cancer in smokers to the odds of cancer in nonsmokers OR = (a/b)/ (c/d) = (ad)/ (bc) The risk ratio (RR), also called the relative risk, is the ratio of the probability of cancer in smokers to the probability of cancer in nonsmokers Given that you know a, b, c, and d, you can compute either ofOdds ratios (OR) are commonly reported in the medical literature as the measure of association between exposure and outcome However, it is relative risk that people more intuitively understand as a measure of association Relative risk can be directly determined in a cohort study by calculating a risk ratio (RR)
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An example of what I am talking about is the choice between risk ratio and odds ratio Odds ratio vs risk ratio You know the difference between risk and odds A risk is the proportion of subjects with an event in a total group of susceptible subjects Thus, we can calculate the risk of having a heart attack among smokers (infarcted smokersThe odds ratio and the relative risk will not always disagree by this much Large effects on groups with high initial risk seem to cause the most problems See Davies et al (1998) for some useful guidelines for when the odds ratio and relative risk are likely to differ When they do differ, the relative risk represents the typical interpretationWhen the disease is rare, the odds ratio will be a very good approximation of the relative risk The more common the disease, the larger is the gap between odds ratio and relative risk In our example above, p wine and p no_wine were 0009 and 0012 respectively, so the odds ratio was a good approximation of the relative risk OR = 0752 and RR = 075
Subsequently, the term relative risk commonly refers to either the risk ratio or the odds ratio However, only under certain conditions does the odds ratio approximate the risk ratio Figure 1 shows that when the incidence of an outcome of interest in the study population is low (Relative risk is the ratio of the risks for an event for the exposure group to the risks for the nonexposure group Attack Rate (Risk) Attack rate for exposed = a ⁄ ab Attack rate for unexposed = c ⁄ cd For this example Risk of tuberculosis among East wing residents = 28 ⁄ 157 = 0178 = 178% Risk of tuberculosis among West wing residents = 4 ⁄ 137 = 0029 = 29% The risk ratio is simply the ratio of these two risks Risk ratio = 178 ⁄ 29 = 61
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) A risk ratio > 1 suggests an increased risk of that outcome in the exposed group A risk ratio < 1 suggests a reduced risk in the exposed group Percent Relative Odds vs relative risk Odds vs relative riskWhen the risk ratio cannot be obtained directly (such as in a casecontrol study), the odds ratio is calculated and often interpreted as if it were the risk ratio Subsequently, the term relative risk commonly refers to either the risk ratio or the odds ratio However, only under certain conditions doesThe odds ratio ((a/c)/(b/d)) looks at the likelihood of an outcome in relation to a characteristic factor In epidemiological terms, the odds ratio is used as a point estimate of the relative risk in retrospective studies Odds ratio is the key statistic for most casecontrol studies
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Both the odds ratio and the relative risk compare the relative likelihood of an event occurring between two groups The relative risk is easier to interpret and is consistent with general intuition Some designs, however, allow only for the calculation of the odds ration Covariate adjustment is easier for an odds ratio Even with initial risks as high as 50% and very large reductions in this risk (odds ratios of about 01), the odds ratio is only 50% smaller than the relative risk (01 for the odds ratio compared with a true value for the relative risk of 02) Odds ratio ended up appearing in Question 2 from the second Primary Exam paper of 08, and therefore a more extensive discussion of OR and risk is carried out in the CICM Primary statistics summaries Relative risk reduction RRR= absolute risk reduction divided by the control group risk
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About Press Copyright Contact us Creators Advertise Developers Terms Privacy Policy & Safety How works Test new features Press Copyright Contact us CreatorsRelative Risk (RR) & Odds Ratio (OR) The difference between odds and probability is important because Relative Risk is calculated with probability and Odds Ratio is calculated with odds Relative Risk (RR) is a ratio of probabilities or put another way it is one probability divided by another Odds Ratio (OR) is a ratio or proportion of oddsRelative Risk Relative risk is a ratio of the risks of two groups In the example described above, it would be the risk of heart attack for a person in their current condition compared to the risk of heart attack if that person were in the normal ranges However, to truly interpret the severity of a relative risk we have to know the baseline risk
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If odds are stated as an A to B chance of winning then the probability of winning is given as P W = A / (A B) while the probability of losing is given as P L = B / (A B) What does Relative Risk tell you?See all my videos at https//wwwzstatisticscom/videos/Health Stats IQ playlisthttps//youtubecom/playlist?list=PLTNMv857s9WUI5YsQMW14trmbopjZMWPa000 Int2) Relative Risk and Odds Ratio for the nonobese Relative Risk and Odds Ratio for the obese 3) Overall, you can see that decreasing the baseline incidence will decrease the odds ratio (300 in those who are nonobese versus 129 in those who are obese) Obviously, these results run counter
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The odds ratio will be greater than the relative risk if the relative risk is greater than one and less than the relative risk otherwise In the example above, if the adjusted odds ratio were interpreted as a relative risk, it would suggest that the risk of antibiotic associated diarrhoea is reduced by 75% for the intervention relative to the placebo group Thus the odds ratio is (a/b) / (c/d) which simplifies to ad/bc This is compared to the relative risk which is (a / (ab)) / (c / (cd)) If the disease condition (event) is rare, then the odds ratio and relative risk may be comparable, but the odds ratio will overestimate the risk if the disease is more commonLet us now look at the relation between the relative risk and the odds ratio (Zhang and Yu, 1998) OR= ˇ 1 1 1ˇ 1 ˇ 2 1 ˇ 2 = ˇ ˇ 2 1 2 1 1 = RR 2 1 (21) From this we see that OR is always further away from 1 than RR But, more importantly, we see that the odds ratio is close to the relative risk if probabilities of the outcome are small (Davies et al, 1998)
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