None:75
75 þ 65¼54%
Minor:160
160 þ 175¼48%
Major:100
100 þ 135¼43%
Death:15
15 þ 25¼38%
The method introduced in this section seems particularly ideal for the evalua-
tion of ordinal risk factors in case–control studies.
Example 6.16 Prematurity, which ranks as the major cause of neonatal mor-
bidity and mortality, has traditionally been defined on the basis of a birth
weight under 2500 g. But this definition encompasses two distinct types of
infants: infants who are small because they are born early, and infants who are
born at or near term but are small because their growth was retarded.Pre-
maturityhas now been replaced bylow birth weightto describe the second type
andpretermto characterize the first type (babies born before 37 weeks of ges-
tation).
A case–control study of the epidemiology of preterm delivery was under-
taken at Yale–New Haven Hospital in Connecticut during 1977 (Berkowitz,
1981). The study population consisted of 175 mothers of singleton preterm
infants and 303 mothers of singleton full-term infants. Table 6.20 gives the
distribution of age of the mother. We have
C¼ð 15 Þð 25 þ 62 þ 122 þ 78 Þþð 22 Þð 62 þ 122 þ 78 Þ
þð 47 Þð 122 þ 78 Þþð 56 Þð 78 Þ
¼ 20 ; 911
D¼ð 16 Þð 22 þ 47 þ 56 þ 35 Þþð 25 Þð 47 þ 56 þ 35 Þ
þð 62 Þð 56 þ 35 Þþð 122 Þð 35 Þ
¼ 15 ; 922TABLE 6.20
Age Cases Controls Total
14–17 15 16 31
18–19 22 25 47
20–24 47 62 109
25–29 56 122 178
b 30 35 78 113
Total 175 303
ORDERED 2kCONTINGENCY TABLES 233