 Let's discuss a couple of calculations. So the odd ratio is used in our case control studies, and it measures the odds of exposure among cases versus the odds of exposure among controls. So the formula used for odds ratio is going to be odds ratio equals A divided by C over B divided by D, which we can rearrange to make this an easier formula to remember is just A times D over B times C as you see here. So let's take our standard 2 by 2 square, and we are going to apply an example. So our top row is going to be positive disease, and our side up and down is going to be exposure. So the first one, they have positive for the disease or they're negative for the disease. And then on the left side, they are positive for exposure or they're negative for exposure. So we're going to take a case where 20 out of 30 lung cancer patients and five out of 25 healthy individuals reported smoking. So 20 patients that have lung cancer reported smoking, five healthy patients that don't have lung cancer reported smoking. And then we have 10 patients that have lung cancer and that have never smoked, and we have 20 healthy patients that never smoked. So we're going to assign each one of these types of patients to a box. And we're going to label these boxes A, B, C, and D. So there are 20 patients with cancer who had an exposure to smoking. That will go into box A. So they are positive for disease and they're positive for exposure. There were five patients that were healthy, had no cancer, but they did have exposure to smoking. So therefore, we're going to put those into box B where they had a negative disease but positive exposure. We had 10 patients who did have cancer but they never had exposure to smoking. So we're going to put those into box C. They're positive for disease, negative for exposure. And finally, the last population had 20 patients with no cancer, no exposure to smoking. Since we've categorized them all now into the four boxes, we can do our calculations. So we have A as 20, multiply that times D, which is 20. And then we divide all of that by B, which is 5, times C, which is 10. We get a calculation of 400 divided by 50, which is equal to 8. So this means that it is 8 times more likely for someone to have cancer that has a history of smoking than to someone who does not have a history of smoking. So our odds of exposure to smoking versus our odds of exposure to a control or nonsmokers. The next calculation we'll discuss is relative risk. Relative risk, as we mentioned previously, is used in cohort studies. Remember that RT in cohort responds to the R and the T of relative. This tells us whether there is a risk of developing a disease in an exposed group divided by the risk in an unexposed group. So the calculation for relative risk is A divided by A plus B all over C divided by C plus D. So we're going to set up our 2 by 2 square again, where we have positive and negative in our disease and positive and negative in our exposure. And once again, A, B, C, and D for labeling our boxes. So let's take another example. We have five patients out of 10 that were exposed to radiation are diagnosed with cancer. And then we have one out of 10 people that were not exposed to any radiation, and those are diagnosed with cancer. So what is our relative risk? Well, we said that we had five patients that were exposed to radiation and diagnosed with cancer. So they have disease and they have exposure. Therefore, they go into box A. We had five patients then that did have exposure to radiation but did not develop disease. So they go into box B. We then have one patient that did have the disease but no exposure to radiation. So that goes into box C. And then finally, the remaining nine patients go into box D where they have no disease and no exposure. So we can take those numbers now, plug them into our formula, and we will see that our relative risk is equal to five divided by five plus five all over one, which is C, divided by one plus nine. So calculating that out, we're going at five over 10 divided by one over 10. This all equals out to 0.5 divided by 0.1. And you do the rest of that math and you end up with five. So in this particular example, there is a five times greater risk of developing cancer with exposure to radiation. Now there is a couple things to note on relative risk. First, if our relative risk is equal to one, then we have no association between our disease and our exposure. If our relative risk is greater than one, whereas in this example, it was five, then that means we do have an increased disease occurrence with that exposure. And then once again, if we have a relative risk that is less than one, that means that exposure actually decreases our risk of that disease.