biostats conecpt!!? - upinthesky

[ArchivalUser]

upinthesky, sorry for the delay, I left to finish what I was doing and I am glad that you had great discussion with xahdum about the topic.

I did not check the book after reading your post, now I see what you meant.

the book gave 2 exples:
-the incidence of shingles is 2000 per 100000 per year or 2%
-the prevalence per year is 170 per 100,000 per year or 0.17%


P= anual incidence X average duration per year

in those 2 exples, what they explained is NEW concept and I am sorry for not checking your reference b4 my post.

Now If P=0.17% , I=2%, P=I x D, then D must be 1/12 of the Incidence in order to get 0.17%, since we know 1 year is 12 months, we can conclude that the disease duration is 1month or 4 weeks.

If the treatment cut the duration to half(2 weeks), then D is 1/24 and the new prevalence decrease from 0.17% to 0.085%.

Sorry again.

[ArchivalUser]

typo

Now If P=0.17% ,and I=2%,
P=I x D, then P(not D) must be 1/12 of the Incidence in order to get 0.17%,
since we know 1 year is 12 months, we can conclude that the disease duration is 1month or 4 weeks.

[ArchivalUser]

ok, back again...
i got all the mathematical stuff okt3... but it was only due to the fact that this equation according to what i understood from the kaplan lectures is not a pure mathematical equation... i mean the whole thing was mainly to state that by increasing the incidence, or increasing the duration, then the prevalence would increase, but it shouldn't be prone to simple mathematical manipulations to get another concept.. for example, if we said duration = prevalence incidence: decreasing the incidence would increase the duration of the disease!! that doens't seem to be reasonble.... that's why, the main use of that equation, based on what've understood, is to show the effect of changes in incidence and duration on prevalence... nothing else..

getting back to my case xahdum: for the incidence, u r right, it is 520 for 91... but i'm still not sure why would we have to subtract the 5 out of 20 to get the denominator for the prevalence, i guess it would be also 520.. 5 are the ppl who have it ( regardless of when did the acquire it, and 20 are the ppl at risk for having it!! i was just looking on a q in kaplan lectures and it seems that they didin't subtract the diseased out of the general pop at risk)

[ArchivalUser]

You are right under prevalence the denominator is = to TOTAL POPULATION