Hello, and welcome back to this global health YouTube channel. My name’s greg marking This is the second in a series of videos that we’re doing on epidemiology and study design and research methods and in this video we’re going to take the quick look at the difference between case control studies and cohort studies and we’re going to have a look at the the relative strengths of each each study design. Let’s start with case control studies. Now to do a case control study where you start is with a group of cases, so people that have a particular condition, they’ve got a disease and and it’s a great study if it’s a rare condition or rare disease, right? So you find people that have a disease let’s say, for example, we found a group of people who at the age of 21 suddenly find their hair goes green. So we collect those people in a group, we create an analogous group, right, people that don’t have that condition But are in every other way similar to the first group right then we look back in time, we look retrospectively at their respective histories and try and identify exposures that may have — and we can look at multiple exposures — that may have led to the condition in this case of green hair and then we compare these two groups the people with green hair and the people without green hair and we try to determine if there is a difference in the extent to which they were exposed to the group of exposures that were interested in. Okay, so to summarize your case control study: you’ve got a group of cases and controls. You’re looking back in time, so it’s retrospective, you can use rare outcomes right, so you’re looking at an outcome and then looking backwards that’s important because when we talk about cohort studies It’s the office [where] run so you’re looking at rare outcomes You’re looking backwards in time, so it’s retrospective you’re talking about looking at multiple exposures, now, importantly you can do it fast right because you can do the study right now you can you can get data today on what happened in yesteryear. And it’s cheap, right doesn’t cost much money to do interestingly or importantly the evidence that you get from a case control study is not considered to be strong evidence. Again now let’s talk about cohort studies. Right and we’re going to I want you to understand the difference between these two studies now remember when we talked about a case control study we started with outcomes of interest sometimes a rare outcome, and we look back in time at multiple exposures. The cohort study is exactly the opposite we start off with an exposure of interest [right], so it might be a rare exposure There’s just a few people in the world that have been exposed to something that’s unusual. People that have been to the North Pole, right so we will find that group of people we find an analogous group we find a second group of people who are in every other way similar to the first group except they haven’t had the exposure of interest like these are people that haven’t been to the North Pole, but in terms of their demography and their age structures and their socio-economic strata, et cetera, et cetera, you try and make sure that they pretty much the same because you’re going to compare these two groups. Now you look forwards in time and you follow this group app right? you follow the cohort app over time to see what outcomes emerge over time as a result of that exposure, right? So just to summarize we’re talking about a prospective study you looking at rare exposures you can examine them for multiple outcomes. It takes a long time to do because you’ve got to follow this group app over time It’s expensive, right, so if you run a study like that it costs a lot of money But what you do [do], what you do get out of it is good strong evidence. So it’s stronger it’s considered to be stronger evidence than a case control study, okay. I hope that was useful. I hope you found that helpful. In the next video we’re going to look at randomized control trials and the idea of confounding. If you’re thinking of subscribing to this channel, let me give you one or two reasons why I think that’s a great idea. Firstly, if you subscribe to the channel you get an email alert whenever there’s a new video posted and so you’ll be up to date and the things that we say and do here on the channel. 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