Incidence is typically calculated as a number diseased per exposed population per unit time, usually year. For each year, divide the people found, by the people exposed. You arrive at figures fr each year being 4%, 5% and 6%, avergae being 5%.

Q: Why not count intake year for which the incidence seems to be 40%?

Ans: So that's the trick of the question. IT seems to imply the intake year also has 4000 new cases, but this isn't incidence, this is prevalence! What happened here was that the population of 10,000 was not being screened for AIDS. Then you started the screening program.

Typically in any screening program, you will establish the baseline prevalence of the disease by testing all of the general population. You find 4000 new cases, but these were just undetected cases. You exclude them from your monitoring. For the next year, you keep a watch on the 6000 left, of which 400 develop AIDS. You calculate incidence for this year as 6.7% and exclude these 400 from the next year and so on.

To get such vague questions right:

1. Consider saying all the statements in the snarkiest tone possible under your breath (translate to mother tongue if IMG for bonus effect).

"Stop Bickering" - while saying, don't you get a hint of paternalism, like the doctor treating the parents as teenagers themselves. They are fully grown-up themselves; they know they shouldn't bicker!

1. Don't choose the refer option on Step 1 (99% of the time correct).... unless no other option at all fits your case first UWorld has questions going to court as the right answer even! But in all of them, it was evident that no other option could come close to being polite/nice/ethical/legally correct.

2. Options that say - encourage/endorse/discuss/ask - are always preferred. Communication = Key Questions