I work in healthcare and there have been multiple times where I’ve seen disease prevalance statistics that include “undiagnosed cases”. If they have not been diagnosed then where do they get those numbers from?
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Let’s pretend there is a disease called Womblyblobitis that affects kindergartners. The schools in the country keep track of how many people have Womblyblobitis so we know that 8 million kids have been reported to have the disease.
BUT, we wonder how many people ACTUALLY have this since some people who have it haven’t been tested so haven’t been reported and counted.
SO – we go to a small number of kindergarten classes and test everybody in the class. We can’t go to every single kindergarten class and test everybody, so we have to do the test on just a few classes.
If we find that in the classes we test, each class had 2 kids with Womblyblobitis, we can then predict that if we were able to test every kid in every class in the country, we only would need to know how many classes of kids there were to predict the number of kids who had the disease without testing everybody.
(yes – epidemiologists, don’t come after me with the details / power / bias / sample size etc – this is ELI5 after all!)
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