Rate Ratio Example:
The Association of Interested Persons held their annual conference during the first week in June. There were
two events: a dinner meeting on Wednesday evening (75 attendees), and a luncheon awards ceremony on Thursday at
noon (60 attendees). Twenty (20) % of the 75 Wednesday dinner participants subsequently developed signs and symptoms of
gastrointestinal illness; 5 of the 60 luncheon participants became ill. Calculate the rate ratio to help determine
which event may have been the source of the illness. The rate ratio is calculated as follows:
1)
Calculate the attack rate for the dinner meeting :
x = number of ill persons attending the dinner meeting
y = number of persons attending the dinner meeting
attack rate = (x/y) x 100 = (20/75) x 100 = 27%
2)
Calculate the attack rate for the luncheon:
x = number of ill persons attending the luncheon
y = number of persons attending the luncheon
attack rate = (x/y) x 100 = (5/60) x 100 = 8%
3)
Calculate the rate ratio:
The dinner meeting attendees were 3.4 times more likely to become ill than those who attended the luncheon.
Now that you know how to calculate and use each type of morbidity rate, you have mastered some important tools
for investigating infectious diseases.
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