Difference between odds ratio and relative risk pdf

Effect of changing incidence on or problem let us consider the relationship between smoking and lung cancer. Relative risk can be directly determined in a cohort study by calculating a risk ratio rr. They have very well explained the difference between odds ratio and relative risk. Relative risks have the appealing feature of summarizing two numbers the risk in one group and the risk in the other into one. Odds ratio, hazard ratio and relative risk 67 8 nurminen, m. The relative risk reduction is derived from the relative risk by subtracting it from one, which is the same as the. The basic difference is that the odds ratio is a ratio of two odds yep, its that obvious whereas the relative risk is a ratio of two probabilities. Tutorial about hazard ratios students 4 best evidence. Can you explain the difference between these two numbers. The medical community has developed various popular measures of the strength of observed effects. When to use the odds ratio or the relative risk int j public health 53. It compares the incidence or risk of an event among those with a specific exposure with those who were not exposed eg. In some sense the relative risk is a more intuitive measure of effect size.

Relative risk, odds ratio, attributable risk and number needed to treat an improved version of this article is now available in third edition 2012 of the book medical biostatistics crc press, new york by abhaya indrayan adapted from basic methods of medical research, third edition. A risk ratio rr, also called relative risk, compares the risk of a health event disease, injury, risk factor, or death among one group with the risk among another group. Odds ratio, relative risk, risk difference statistics. Some studies use relative risks rrs to describe results. Understanding relative risk, odds ratio, and related terms. The relative risk also known as risk ratio rr is the ratio of risk of an event in one group e.

Attributable risk an overview sciencedirect topics. In other words, the attributable risk is the extent to which the. The relative risk is easier to interpret, so the odds ratio alone is not very helpful. The relative risk rr 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. Odds are expressed in the ratio, the probability is either written in percentage form or in decimal. The relative risk also known as risk ratio rr is the ratio of risk of an event in one. 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. The attributable risk risk difference or attributable rate is the difference between the incidence of disease in the exposed population versus incidence in the unexposed population see table 217. Odds ratios refer to a ratio between members within a population expressing a. Since it is a ratio of ratios, the odds ratio is very difficult to interpret. Relative risk and odds ratio european public health.

Relative risk and odds ratios university of guelph atrium. There is some confusion about the use of the odds ratio versus the relative risk. Dec 14, 2015 odds ratios versus relative risk published on december 14, 2015 by howard herrell, md many great things have been written about the difference between odds ratios or and relative risks rr. Relative risk and odds ratios examples university of guelph atrium. The question of which statistic to use is subtle but very important. Relative risks and odds ratios statistics 149 spring 2006. The relative risk and the odds ratio are measures of association between exposure status and disease outcome in a population. Difference between probability and odds compare the. May 15, 2014 the difference between odds ratio and risk ratio. Odds ratio versus relative risk created by 20010109 dear professor mean. The relative risk rr and the odds ratio or are the two most widely used measures of association in epidemiology. Jun 10, 2014 the difference between odds and probability is important because relative risk is calculated with probability and odds ratio is calculated with odds. Both measure an association between a risk factor and an outcome, but relative risk is related to the difference in outcomes with and without a risk factor, and odds ratio is related to the difference in the. Odds ratio versus relative risk odds ratio can be calculated in a cohort study and in a casecontrol study.

From this contingency table, we can calculate an odds ratio and likelihood ratio. The odds ratio or is the ratio of odds of an event in one group versus the odds of the event in the other group. It compares the incidence or risk of an event among those with a specific exposure with those who were not exposed eg, myocardial infarctions in those who smoke cigarettes compared with those who do not figure. Same example but now the probability of death in the control group is 0. To use this calculator, you need enter the data for two groups in the text boxes below. In the city with relaxed gun laws, there were 50 shootings in a. T o use or not to use the odds ratio in epidemiologic studies. Together with risk difference and odds ratio, relative risk measures the association between the exposure and the. The relative risk is most often used, especially in studies showing the effects of a treatment.

Relative risk, risk difference and odds ratio computational. Examples of measures of association include risk ratio relative risk, rate ratio, odds ratio, and proportionate mortality ratio. To my understanding about these factors of association, i would like to add one thing also that is time. The odds ratio is the only parameter that can be used to compare two groups of binary responses from retrospective studies. It is essential to understand that odds ratio is not the same as relative risk, and relative risk is not the same as odds ratio. The other day i was emailing with a statistical genetics colleague about a rare snp associated with a phenotype. The risk difference also gets referred to as the attributable risk. The relative merits of risk ratios and odds ratios research. The risk difference is an absolute measure of effect i. However, there are certain commonly occurring situations in which the estimate of the relative risk is not very good and the odds ratio can be used. With odds ratios and relative risks, we dont have a sense of the magnitude. The odds ratio and the relative risk will not always disagree by this much.

The exposure odds ratio is equal to the disease odds ratio relative risk can only be calculated in a cohort study. Probability is the likelihood of an event in relation to all possible events. Both are two different statistical concepts, although so much related to each other. The general rule though is that if the prevalence of the disease is relative risk and the odds ratio will be approximately the same. This post tries to explain the difference between odds ratios and relative risk ratios. For the presentation of risk, both relative and absolute measures can be used. Sometimes, we see the log odds ratio instead of the odds ratio. What is the difference between odds ratio and relative risk. As simple as it can get chittaranjan andrade, md abstract risk, and related measures of effect size for categorical outcomes such as relative risks and odds ratios, are frequently presented in research articles. Probability ensures that an event will occur, but odds is used to find out whether the event will ever occur. The relative risk rr is the risk of the event in an experimental group relative to that in a control group. In the population under investigation, relative risk refers to a ratio between members of the population expressing the trait of interest e. In the population under investigation, relative risk refers to a ratio between members of the population expressing the trait of interest.

The direct computation of relative risks is feasible if meaningful prevalences. The primary difference between odds and probability is that while odds is a ratio of occurrence to nonoccurrence, the probability is the ratio of occurrence to the whole. Odds ratios cannot be calculated directly from relative risk ratios. See davies et al 1998 for some useful guidelines for when the odds ratio and relative risk are likely to differ. In contrast, hazard ratio takes account not only of the. Differences between statistical approaches can be more pronounced with other data. When two groups are under study or observation, you can use two measures to describe the comparative likelihood of an event happening. This work is licensed under a creative commons attribution.

When the data to be analyzed consist of counts in a crossclassification of two groups or conditions and two outcomes, the data can be represented in a fourfold table as follows. However, it is relative risk that people more intuitively understand as a measure of association. The comparison of odds extends nicely to regression analysis e. In this scenario, heart attack is the event of interest bad outcome, and the relative risk and odds ratio measures allow you to compare how likely the members of each group are to suffer the bad outcome. Since relative risk is a more intuitive measure of effectiveness, the distinction is important especially in cases of medium to high probabilities. Odds ratios or are commonly reported in the medical literature as the measure of association between exposure and outcome. When they do differ, the relative risk represents the typical interpretation. There are different ways of doing this for different kinds of data and sizes of table, but two are particularly important in health research. Examples calculating relative risk calculating relative risk imagine that the incidence of gun violence is compared in two cities, one with relaxed gun laws a, the other with strict gun laws b.

Every medical student at some point has been taught the difference. Both the odds ratio and the relative risk compare the likelihood of an event between two groups. Relative risk and odds ratio the relative risk rr is the probability that a member of an exposed group will develop a disease relative to the probability that a member of an unexposed group will develop that same disease. The odds ratio or is the odds of an event in an experimental group relative to that in a control group. Wittum, in equine internal medicine second edition, 2004. The absolute risk is the probability of an event in a sample or population of interest. Yet these statistical terms are confused and misused every day in both the writing of and the interpretation of literature which well talk more about at the end if you get bored of the. You tell me what this means if this is the way you think about the likelihood of outcomes in everyday life.

The relative merits of risk ratios and odds ratios. The relative risk of a treatment is the ratio of risks of the treated group and the control group, also called the risk ratio. It is computed as, where is the incidence in the exposed group, and is the incidence in the unexposed group. Proportions, risk ratios and odds ratios risk ratio or relative risk chisquared tests are tests of significance, they do not provide estimates of the strength of relationships. The measures of effect are generally expressed as relative risks and odds ratios. Odds ratio, relative risk, and risk difference are all slightly different ways of. When the outcome of interest is relatively rare odds ratio and relative risk will be very close in magnitude. For more than 20 years, there has been debate about the relative merits of risk ratios compared with odds ratios as estimates of causal associations between an exposure such as smoking or medication for high blood pressure and a binary outcome such as death vs life. Jul 31, 2011 what is the difference between probability and odds. Many great things have been written about the difference between odds ratios or and relative risks rr. Rr is based upon the incidence of an event given that we already know the study partic. If the relative risk is 1, the tutoring made no difference at all. Relative risk and odds ratios what is relative risk. One of the main differences between risk ratio and hazard ratio is that risk ratio does not care about the timing of the event but only about the occurrence of the event by the end of the study i.

The odds ratio is extremely important, however, as it is the only measure of effect that can be computed in a casecontrol study design. Probability is expressed as a number between 0 and 1, while odds is expressed as a ratio. Difference between odds and probability with comparison. Therefore it may be useful to cross check results, for instance by comparing. 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 nonexposed group. The odds ratio is the ratio of the odds of an event in the treatment group to the odds of an event in the control group.