Denominator in rate


This isn't so much a statistical question as it is just figuring out the appropriate dates in which to include cases into a denominator for a rate. Well, I guess it is kind of statistical, as I don't want to over or underestimate my rate, so please allow me to explain!

I want to get a rate of an event that occurs between a specific time frame, our fiscal year, Apr 1 to Mar 31 2012.

So right away, my numerator is # events occurring in Apr 1 2011 - Mar 31 2012. This is great, and I can determine these cases directly looking at the data.

My denominator should be consist of all the subjects that were eligible to have the event occur in the same time frame. The event is a bit complicated however, let me just explain it.

If the event occurs for someone, it should occur between 18 - 24 months of of their life (working with developmental stuff here).

So assume for these infants, I have their birthdays. In order to form a denominator to include all eligible children for the rate, how should I set the maximum and minimum dates of birth for inclusion?

Again, I've been trying to wrap my head around this and here are the details:

Infants - I have date of birth, and if they have the event between 18-24 months of life during Apr 1 2011 and Mar 31 2012, they are a "case"

I want "eligible" children for the event as the denominator. In order to be eligible, they should be able to get the event at any time through out Apr 1 2011 - Mar 31 2012. How can I set my denominator (im guessing based on bdays)?

Thanks in advanced!
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