differentiate incidental rate from prevalence rate?

Answer 1

Here is a good website that compares incidence rate with prevalence rate:

http://www.umanitoba.ca/centres/mchp/concept/dict/Prev_incid/prev_incidence.html

Note that it's almost universally called an incidence rate, not an 'incidental' rate. That might help you with your web searches. Good luck.