 Welcome back to the revision series in dentistry and more. So today's topic is relative risk and odds ratio. So it is the most confusing part of epidemiology and the calculation is also a little bit tricky because the relative risk is seen in cohort study that is a prospective study or a forward looking or longitudinal study and odds ratio we calculate in case control study. So always relative risk is the best method of our best assessment and that will be in cohort study and this will be in case control study or ratio will be case control study. Cohort study is like from the beginning there is no disease. So as the disease progresses the diseases or the people with disease starts appearing so it is very easy to understand the actual risk. But in case control study we are going backward it is a reverse looking study or backward looking study or a retrospective study. So actually when the study starts there is actually cases so we just compare it with controls. So odds ratio is nothing but an indirect measure of risk estimation. So in odds ratio it is not a direct measure always relative risk is best. But most of the time relative risk estimation is very difficult because we know the difficulties of prospective study so always we go for odds ratio. So let's see how do we calculate relative risk. So always we should make a contingency table that is 2 by 2 contingency table and always put on x-axis disease and no disease on y-axis exposed and not exposed and go like this A B C D don't change this alphabet order. So relative risk is the formula is incidence among exposed divided by incidence among non-exposed. So incidence is the number of new cases that is A among exposed the total number of exposed is horizontal that is A plus B so this is exposed to people A and B. So A by A plus B is the incidence among exposed and incidence among non-exposed okay so incidence is A and C so C is a non-exposed group so C and total non-exposed that is C plus D so C by C plus D so relative risk is A by A plus B divided by C by C plus D. So we get applied. So whereas in case control study as I mentioned earlier there is no incidence incidence is new cases it is a prevalent study that is cases already present. So what here checking is the exposure rate that is how many of the cases and how many of the controls were exposed to the particular risk factor if we are checking smoking and lung cancer when the study starts we having lung cancer patients and people without lung cancer so we are asking them or we are trying to find out how many of the patients and controls were exposed to smoking here it is not like that when we start the study there is no people with people are not having any disease so as a disease progress we are trying to find out how many are how many of them are getting lung cancer okay that is a difference here people starts with the habit or without habit but there is no disease but here the disease is present already so we are checking how many of them are having exposure that how many of them are having smoking habit. So here the formula is odds ratio is odds of exposure in cases or among cases or odds of exposure among controls so odds of exposure that is A by C this is the exposure and odds of exposure A by C odds we are checking odds A by C is the odds of exposure among cases and also of exposure among controls that is B by D so odds of exposure among controls B by D okay this is not exposed so odds of exposure means B by D so odds ratio is odds of exposure among cases divided by odds of exposure among controls that means A by C divided by B by D so we take cross product it will become AD by BC okay so we can directly go for AD by BC but this is how we get we are checking odds of exposure among cases and odds of exposure among controls here we are checking incidence among exposed and incidence among non-exposed that is the difference now let's see one example and you get a better idea so let's see an example so very hypothetical example so we take hundred exposed people that means hundred people are smoking and 70 got disease 30 did not get and hundred people without the habit out of them 10 got disease 90 are not with disease so horizontally exposure present or absent this is disease present or absent so we always keep a comparison group to prove the relative risk or whatever risk estimation we need a comparison group without comparison group we cannot prove anything so that is A by A plus B divided by C by C plus T that is A is 70 A plus B is 100 70 plus 30 C is 10 and C plus T is 10 plus 90 so 100 70 by 100 divided by 10 by 100 so 70 by 100 into 100 by 10 so we get 7 so that is a relative risk 7 that means the chance of getting lung cancer is 7 times higher in exposed to group compared to the non-exposed group well coming to odds ratio we have hundred exposed and hundred non-exposed but the thing is we have 80 cases and 120 controls because the study starts with case and controls because it is a case control study so 80 cases and 120 controls we are taken so among them we are assessing how many or how many are exposed and how many are not exposed so odds ratio is the comparison of exposure among cases and exposure among controls that is odds of exposure among cases and also of exposure among controls that is A by C also of exposure among cases and B by D that is also of exposure among controls that is 70 by 10 into 30 by 90 when we multiply or cross product we take 70 into 90 divided by 30 into 10 so we get 21 that means the strength of association between smoking and lung cancer is 21 times or we can say that people with disease that is lung cancer the chances of exposure were 21 times compared to the people without lung cancer we can directly say people with smoking has a 21 times chance of getting lung cancer compared to people without lung cancer so it is an indirect measure this is the direct measure we can directly say the chances seven times greater among the non-exposed group this is an indirect measure we are actually checking the exposure how many of them were exposed how many of them were having that habit so this is an indirect measure this is a direct measure so that is how we calculate relative risk and odds ratio both are indication of risk about causal factor and outcome so I'll come up with another revision series in dentist channel thank you