Question

The investigators administered the study questionnaire by telephone from March 15-23. Students selected for participation were called at their dormitory room or their home telephone number as recorded in university records. If the student was not present at either location but information on his/her whereabouts was available, additional phone calls were made to contact the student. Students not reached during spring break were interviewed on their return to the university.

Thirty-six cases and 144 controls were contacted. Cases included in the study were similar to all cases with respect to gender, age, and year in college. Their dates of onset of illness had a distribution similar to that seen in Figure 1.

Figure 1. Onset of gastroenteritis among students, University X, Texas, March 1998. (N-72) (Date of onset was not known for three ill students.) 45-т 40 35 O 25 o 20 5 15 10 5 6 7 8 10 11 12 Date of Onset (March)

Results from the unmatched case-control study were tabulated by TDH and CDC epidemiologists. Only persons who ate at the main cafeteria for the specified period were included in the meal-specific analyses. (Table 2)

Table 2. Risk factors for illness among persons eating at the main cafeteria, unmatched case-control study, University X, Texas, March 9-10, 1998.

Exposure	Ill exposed/ Total ill	Well exposed/ Total well	Measure of association	p-value
Ate at salad bar - lunch March 9	9/30	36/100
Ate at salad bar - dinner March 9	5/18	15/59
Ate at salad bar - lunch March 10	6/28	23/96
Ate at salad bar - dinner March 10	6/15	12/44
Ate at salad bar*	13/19	49/69
Ate at deli bar - lunch March 9	18/30	12/101
Ate at deli bar - dinner March 9	7/18	5/61
Ate at deli bar - lunch March 10	13/29	12/96
Ate at deli bar - dinner March 10	4/16	4/44
Ate at deli bar*	28/36	20/116

Question 20: Calculate the appropriate measure of association for these exposures. Interpret the results.

HI! What is the formula for measuring association?

Answer 1

Hi,

In this question ,we will calculating risk ratio as the measure of association. Risk ratio is defined as, in this case, the ratio of attack ratios of those who are ill and those who are well.

Attack ratio is given as:

a. For ill, attack ratio_ill= Number of ill people exposed/Total number of ill people

b. For well people, attack ratio_well= Number of well people exposed/Total number of well people

Risk ratio = attack ratio_ill/attack ratio_well

Exposure	Ill exposed/ Total ill	Well exposed/ Total well	Measure of association
Ate at salad bar - lunch March 9	0.30	0.36	0.833
Ate at salad bar - dinner March 9	0.28	0.25	1.093
Ate at salad bar - lunch March 10	0.21	0.24	0.894
Ate at salad bar - dinner March 10	0.40	0.27	1.467
Ate at salad bar*	0.68	0.71	0.963
Ate at deli bar - lunch March 9	0.60	0.12	5.050
Ate at deli bar - dinner March 9	0.39	0.08	4.744
Ate at deli bar - lunch March 10	0.45	0.13	3.586
Ate at deli bar - dinner March 10	0.25	0.09	2.750
Ate at deli bar*	0.78	0.09	9.022