Question

After the first three cases were culture confirmed, the management at the factory requested that workers report any symptoms of illness that they might have experienced during the month of July. Of the 3155 employees, 74 reported gastrointestinal symptoms (i.e., diarrhea) in July and stool samples were obtained from these workers. Of the 74 workers who reported that they were ill, 56 were culture-confirmed. Based on retrospective interviews, the onset dates for diarrhea for these 56 workers were as follows:

Date of Onset	Number Cases
July 15	3
July 16	6
July 17	8
July 18	10
July 19	13
July 20	6
July 21	4
July 22	6

1. What was the incidence rate for this eight day period?  
2. List two plausible reasons why only 56 of the 74 workers who said that they had experienced gastrointestinal problems could be culture-confirmed as being infected by e coli O157:H7.

Answer 1

The above table gives the number of employees confirmed with e coli O157:H7 infections over a period of 8 days starting from 15 Jul to 22 Jul. Incidence rate is calculated for each date starting from Jul 15 upto Jul 22. Incidence rate denotes the number of people under risk among the total set of population. Here the total population is 3115, and the cumulative people affected on a date (sum of persons affected each date) is divided by 3115 to calculate the incidence rate. The detailed calculation of incidence rate table is shown below.

Date Onset	Number Cases	Incidence Rate %
15-Jul	3	0.095
16-Jul	6	0.285
17-Jul	8	0.539
18-Jul	10	0.856
19-Jul	13	1.268
20-Jul	6	1.458
21-Jul	4	1.585
22-Jul	6	1.775

Two possible reason why only 56 of the 74 workers tested was culture confirmed might be

1. They might have had Diarrhea or related illness due to other reason (such as food contamination)

2. It might be due to fear of illness. When a group of people is affected, some of them have the feel that they might also been affected. Upon testing those might have been tested negative for e coli.