USMLE About Incidence rate

[DrPettans]

Hi everyone!

I'm confused here:
As far as I know, Incidence rate is number of NEW events in a specific period of time/ Number of people EXPOSED to risk during that specific period of time.

Then as an example, if 5 women are diagnosed with depression over the course of one month, out of a total study population of 100, we could say that 5/100= 0.05 women-month is the incidence here, right? I get that. So far, so good...but here's the tricky part:

We have a population of 100 kids. 25 develop bronchitis. 12 were infected in April and 13 were infected in May, and we want to know what is the incidence rate of bronchitis in that population for those 2 months.

Now, apparently, we cannot just say: 25/100= 0,25 kids-month. I wonder why, but not just that...
I'm having trouble with the denominator.

The correct answer seems to be: 25 new cases / [(100 kids at risk at the beginning of April + 75 kids at risk at the end of May)/2] x 2 months = 25 new cases /175 kids-months= 14% of the kids are getting bronchitis each month.

why?????

Why can't it be :
25 new cases / [100 (which is population at risk at the beginning of April) + 88 (which is 100-12= kids at risk during october, because those already infected are no longer at risk)]/2 months

And if we have to calculate risk at the beggining of the period + people at risk at the end (beggining of April + end of May), why didn't we have to do the same for the 1st example about women developing depression within a month? (which would be:[100 (beggining of the month) + 95 (end of the month)]/ 2
Maybe it is a silly question but I find it very confusing. Any help here is appreciated.

Thank you very much everyone (this is my first post ever).

---

Members don't see this ad.

 

[tasar1898]

I really think that you should not get bogged down with these statistics/incidence and just accept as it is... But for the sake of discussion: The depression rate you are referring to is a cumulative incidence ( people who get disease/ people at risk ) at a PREDEFINED period of time (e.g. you decide to do a study for diabetes in a Texas village that has a totally healthy population of a 1000 people , and over this year 100 develop diabetes , the CUMULATIVE incidence is 100/1000 and is reported as persons for this year only ) On the other hand , the bronchitis rate is an INCIDENCE RATE that is Incidence/Time and is expressed as persons-time for whatever period of time you want . Usually it is reported as persons-months or persons-years , depending on the study. The persons-time is the total time you observed the population at risk , I ll attach a picture so you will probably get it , but I doubt they ll ask anything more than a simple cumulative incidence...

Sorry for the rotation , just cant fix it !

 

[DrPettans]

Ohhh I think i get it now. The book wasn't very claryfing, it was confusing. Thanks for your time and pictures, now my neck hurts but I'm wiser!
haha kiddin. Thank you

 

[tasar1898]

haha , the sacrifices doctors make... no prob and good luck with ur test

 

[worldbeater]

Incidence is all new cases, prevalence is all cases. Just knowing that small bit of info will help you sort out these questions on UWorld. They will play with lead time sometimes, and then will ask you how the incidence and prevalence changes. (Answer: incidence stays the same, prevalence will increase)