Incidence and prevalence

let's talk about measures of disease we're going to start with incidence and prevalence and why don't we start with the definitions of these two terms so incidence is the number of new cases that have occurred in a given time period over the number of people at risk in that given time period and prevalence is the number of cases over the number of total people and so you can see two differences already between these definitions instance looks at the number of new cases so if someone already has the disease we don't care we only care about the ones who newly develop the disease that's different than prevalence in which we are worried about if you already have the disease even if you got it 20 years ago before we were doing our study the account and so that's what these number of cases are counts in here now the other thing that's different is the this time frame aspect here so it's in a given time frame but if we don't have that time aspect here so what that means is that incidence is really a rate a rate has some sort of time inherent in it it's how fast something is happening and so that's how we have the time frame here and prevalence is a proportion and so proportion is a percentage right so what is the percentage of people who have the disease overall the people in total and because of that what we can say is that incidence is a rate so what we're doing is we're really measuring how fast this disease is occurring how fast that's happening and that is in essence a measure of risk the faster the disease is occurring the riskier it is for those people on the other hand prevalence is asks how much of the disease is there in that population is it just a little bit or is it a lot and that makes a difference because that tells us the burden of disease meaning how many people are sick with that disease and how many people do have to care for so that might matter for public policy you might need to build more clinics or build more hospitals to care for those people with the disease now let's look at its edits and prevalence in a little bit more detail so I put our definitions for incidence and prevalence up here and now let's pretend that we're doing a study and this study is going to last 10 years and so here's a timeline representing those 10 years and so here's the time frame of our study represented by this big pink box and what we're going to look at in this study is the onset of cancer in these news people and so here's this line represents one person the old stands for the onset of the cancer and then this little death face represents that they died of the cancer so we got a bunch of people in the study and so here are all the people in the study so here's our initial guy who got cancer about your behalf and then she died around year seven this one got cancer on year one and a half and then died around your nine this person didn't live that long cancer around four died around five this person got cancer before the study but died within the study here's another person who got cancer after the study and died a person who got cancer but then lived through and hadn't died yet and this person you know got it and died before the study and here's a person who never even got cancer so let's look at incidence first so incidence is the number of new cases in a given time frame and so what is our time frame here it's this 10-year period how many new cases well what's the case how are we defining that here we're defining case as the onset of cancer so what we want to know is how many people got cancer during this time period this guy did this one did this one did this one did get cancer but not within the time period so this one doesn't count this one also got cancer but not within the time period this one did this one got cancer but not within the time period and this guy never got cancer so how many new cases do we have we have one two three four now we have to ask how many people were at risk of getting cancer in this time period so this person was right because it from up til here and here they were at risk this person wasn't this person was this person wasn't because they already had cancer by the time this study started so they were never at risk of getting cancer because they already had it this person was at risk of getting cancer they just didn't get it during that timeframe this person was at risk too right because as they head to the study I didn't have it then they did get it this person was never at risk of getting cancer because they died before the thing started and this person was so how many people were really at risk 1 2 3 4 5 6 so we have six people who are at risk so in this particular period our incidence of cancer was 0.66 over this 10-year period 0.66 now let's take a look let's take a look at prevalence now prevalence is like a snapshot in time and so let's pick a particular time point and so we picked a snapshot in time going to look at this it looks like year four we want to see what are the total number of cases so who has cancer at this point in time well this guy does this one does this one does this one does not because they died before then and this one doesn't this one does this one does not and this one does not so how many total cancer cases do we possibly have we have one two three four so we have four cases again right one two three four now what goes in the denominator well the denominator is any one that anyone else that was there right so we have one person two three this is another one for they didn't get cancer till later but tastes count four five six so we have six people that we are looking at in total at this snapshot in time remember this guy didn't live to that snapshot or did this so six or you could say the probe disease and year four in this study was 66% and so this gives you a little bit of an idea of how prevalence and incidents are different all right we're going to talk in some more detail about incidents and different kinds of events and incidents in different ways of measuring incidents in the next video thanks bye

Channel: Rahul Patwari

Views: 154,202

Rating: 4.9382887 out of 5

Keywords: EBM, Incidence, Prevalence

Id: Xd01PXsUdBc

Length: 7min 3sec (423 seconds)

Published: Thu Jul 25 2013