 
* biostat kaplan question help!!!!! 
 #862308 




A population at risk is composed of 100 medical students. 25 medical students develop symptoms of campylobacter infection and are confirmed by lab testing to have it. If 12 students developed campylobacter in September and 13 developed campylobacter in October, what is the incidence rate of campylobacter for those 2 months?
Numerator:25 new cases.
Denominator (persontime at risk) could be calculated by:
[(100 students at risk at the beginning of Sept. + 75 students at risk at the end of Oct.)/2]x2
= [(175/2)x2] months.
= 175 personmonths at risk.
NOW MY QUESTION: THE DENOMINATOR, I HAVE TO ADD BOTH GROUPS AT RISK TO GET THE POP AT RISK FOR THOSE 2 MONTHS (100+75). WHY I HAVE TO DIVIDE BY 2 AND MULTIPLY AFTER BY 2? WHAT ARE THOSE 2s?
Please and thank you. 

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