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I had to solve the following problem. Newborn screening has been implemented for a very rare pathology. For 6 months, and out of 50,000 births observed, no sick case was observed.

What is the prevalence of the disease studied?

The answer is less than 3 in 50.0000.

I do not understand at all where this answer comes from. The prevalence is however calculated as the incidence multiplied by time, but the incidence here is zero!

My idea is to use the fact that incidence is below 1/50000 on 6 months and to then calculate prevalence. Thanks for any suggestion !

Flora Grappelli
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  • This sounds like it is homework or a type of self-study question. If so, please read our [`self-study` info](https://stats.stackexchange.com/tags/self-study/info) for how we handle such questions, and add that tag to your question. – EdM Jan 18 '21 at 21:38
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    If you search for the "rule of three" (add "binomial" if you search for this using Google), this should point you in (what I would guess to be) the right direction. – user215517 Jan 19 '21 at 01:38
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    I assume the question likely understands "prevalence" as the "prevalence in newborns". If it is a rare disease why did you implement the screening? Probably because it is a very severe disease. It it is such a severe disease positive babies would likely die earlier so you would need data an the life expectancy of positive babies and the rest of the population, both of which is not given. If I am right and the question is about the prevalence among newborns there is probably not much point in multiplying anything by time. – Bernhard Jan 19 '21 at 07:05
  • @user215517 Thanks a lot to have quoted me the "rule of three", it is something I've never heard before. Is it widely accepted for such case where no 'sick case is observed? – Flora Grappelli Jan 19 '21 at 08:36
  • @Bernhard Thanks a lot for this very interesting remark. So, if I got it right, from your comment, I can conclude that from the given data, we have directly the prevalence and not the incidence since we do not have details on follow-up on the newborns – Flora Grappelli Jan 19 '21 at 08:39
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    That is how I interpret the question. Doesn't mean it's true. – Bernhard Jan 19 '21 at 09:16

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