In a seroepidemiologic survey of health workers representing a spectrum of exposure to blood and patients with hepatitis B virus (HBV), it was found that infection increased as a function of contact. Table E1.36 provides data for hospital workers with uniform socioeconomic status at an urban teaching hospital in Boston, Massachusetts.
|
TABLE E1.36
|
|
|
Personnel
|
Exposure
|
n
|
HBV Positive
|
|
Physicians
|
Frequent
|
81
|
17
|
|
|
Infrequent
|
89
|
7
|
|
Nurses
|
Frequent
|
104
|
22
|
|
|
Infrequent
|
126
|
11
|
(a) Calculate the proportion of HBV-positive workers in each subgroup.
(b) Calculate the odds ratios associated with frequent contacts (com- pared to infrequent contacts). Do this separately for physicians and nurses.
(c) Compare the two ratios obtained in part (b). A large difference would indicate a three-term interaction or effect modification, where frequent effects are different for physicians and nurses.
(d) Assuming that the odds ratios for the two groups, physicians and nurses, are equal (in other words, type of personnel is not an effect modifier), calculate the Mantel-Haenszel estimate of this common odds ratio.