It can be seen that the rate of employment for the cases (18.0%) was higher
than that for the controls (14.7%), but the di¤erence is not statistically signifi-
cant at the 0.05 level (z¼ 0 : 64 < 1 :65).
Example 6.7 The role of smoking in the etiology of pancreatitis has been rec-
ognized for many years. To provide estimates of the quantitative significance of
these factors, a hospital-based study was carried out in eastern Massachusetts
and Rhode Island between 1975 and 1979. Ninety-eight patients who had a
hospital discharge diagnosis of pancreatitis were included in this unmatched
case–control study. The control group consisted of 451 patients admitted for
diseases other than those of the pancreas and biliary tract. Risk factor infor-
mation was obtained from a standardized interview with each subject, con-
ducted by a trained interviewer.
Some data for the males are shown in Table 6.7. With currently smoking
being the exposure, we have for the cases,
p 2 ¼38
53
¼ 0 : 717
and for the controls,
p 1 ¼81
217
¼ 0 : 373
An application of the procedure yields a pooled proportion of
p¼38 þ 81
53 þ 217
¼ 0 : 441leading to
z¼0 : 717 0 : 373
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ð 0 : 441 Þð 0 : 559 Þ 531 þ 2171q ¼ 4 : 52
TABLE 6.7
Use of Cigarettes Cases Controls
Current smokers 38 81
Never or ex-smokers 15 136
Total 53 217COMPARISON OF TWO PROPORTIONS 217