Incidence Rate for an Event

Question

Hi,

I'm unsure whether I'm oversimplifying this problem.

A zoo records 94,000 visits annually from the residents of a particular region. The region has a population of 67,000 inhabitants. From this, calculate the incidence rate that any individual within the population visits the zoo at any given month of the year.

Assume no multiple visits within a monthly period.

Calculation:

94,000 / 67,000 = 1.4

1.4 is the average visit rate per resident

1.4 / 12 = .12

Is the incidence rate that an individual from the population visits the zoo on any given month therefore 12%?

Can anybody please provide a bit of guidance?

Much appreciated.

Answer 1

Hi John T.,

you've correctly calculated P that a given individual from the population visits the zoo on a given month, assuming random distribution of single visits max per month. But you said, "an individual" which is vague. The problem did ask, "any individual", however, which is "Quite a Different Beast", so to speak. Technically, that means quite exactly any individual (in logical terms the broadest inclusion is thus triggered); the negation of that is that no individual visits the zoo on the given month. So, per strict logic again, you would have to calculate (1-0.12)^67000 [i.e., P(particular individual not visiting, over all individuals)] ~ 5.9*10^-3820. That's only approximate; actually you should calculate on the daily basis rather than monthly, but you get the idea.

Shame on any instructor for using ordinary casual English, when exact terminology is available (Do you see how the "any" generalizes, hmmm?) !