A residency program organized a barbecue at the beginning of the academic year to welcome the new class of first-year residents. The total number of attendees was 100, including faculty, administrative staff, and residents. Hamburgers, hotdogs, and potato salad were served. The following day, 28 of the attendees had diarrhea and vomiting. All of the attendees were questioned about what they had eaten, and the following table was obtained: Which of the following best describes the attack rate among all of the attendees who had potato salad?

Question

A residency program organized a barbecue at the beginning of the academic year to welcome the new class of first-year residents.  The total number of attendees was 100, including faculty, administrative staff, and residents.  Hamburgers, hotdogs, and potato salad were served.  The following day, 28 of the attendees had diarrhea and vomiting.  All of the attendees were questioned about what they had eaten, and the following table was obtained:  Which of the following best describes the attack rate among all of the attendees who had potato salad?

Quizplus · Accepted Answer

There appears to be an outbreak as almost a third of the attendees (28/100) developed diarrhea and vomiting; this is clearly a larger number than expected. The attack rate is the ratio of the number of individuals who become ill divided by the number of individuals who are at risk of contracting that illness; it is often used in outbreak investigations. The attack rate for potato salad corresponds to the proportion of attendees who became ill out of the total number of attendees who had potato salad (alone or in combination with other food items). Among those, the number who became ill was 25 (3 out of the 10 who had potato salad only + 5 out of the 25 who had hamburgers and potato salad + 3 out of the 8 who had hotdogs and potato salad + 14 out of the 30 who had hamburgers, hot dogs, and potato salad = 25). Therefore, the attack rate for potato salad was: (25/73) × 100 = 34%. (Choice B) There were 27 attendees who did not eat potato salad: the 15 who had hamburgers only and the 12 who had hotdogs only. Of those, a total of 3 (2 out of the 15 who ate hamburgers only + 1 out of the 12 who ate hotdogs only = 3) became ill. Therefore, the attack rate among those who did not eat potato salad was: (3/27) × 100 = 11%. Despite the fact that these individuals only ate hotdogs and hamburgers, a percentage of them still became ill, probably as a result of some contamination of these food items with the potato salad. For the sake of completion, the attack rates for the other individual food items or combinations of food items are shown in the following table (they do not need to be calculated to arrive at the correct answer): 11ec160b_3704_b765_9249_9d6a30179122_MD0001_00 The food item responsible for an outbreak often (though not always) has the largest attack rate and the largest difference in attack rates between those who consumed the item and those who did not. In this example, as calculated above, the difference in attack rate between those who had potato salad and those who did not was: 34% − 11% = 23%. As seen in the exhibit, the differences in attack rates were smaller for hamburgers (7%) and hotdogs (16%). It is possible that potato salad is the source of the outbreak. Educational objective: The attack rate is the ratio of the number of people who contract an illness divided by the number of people who are at risk of contracting that illness.

A Residency Program Organized a Barbecue at the Beginning of the Academic