A common symptom of otitis media in young children is the prolonged presence of fluid in the middle ear, known as middle-ear effusion. The presence of fluid may result in temporary hearing loss and interfere with normal learning skills in the first 2 years of life. One hypothesis is that babies who are breastfed for at least 1 month build up some immunity against the effects of the infection and have less prolonged effusion than do bottle-fed babies. A small study of 24 pairs of babies is set up, in which the babies are matched on a one-to-one basis according to age, sex, socioeconomic status, and type of medications taken. One member of the matched pair is a breastfed baby, and the other member is a bottle-fed baby. The outcome variable is the duration of middle-ear effusion after the first episode of otitis media. The results are shown below

TABLE 9.11 Duration of middle-ear effusion in breastfed and bottle-fed babies Duration of Duration of effusion in effusion in breastfed bottle-fed Pair number baby (days) baby (days) 1 2O 1 8 2 1 1 35 3 3 '7 4 24 182 5 '7 6 6 28 33 '7 58 223 8 7 '7 9 39 5'7 10 1 '7 76 1 1 1 '7 186 1 2 1 2 29 13 52 39 1 4 14 1 5 15 1 2 21 16 3O 28 1 7 7 8 1 8 15 2'7 19 65 7'7 20 1O 1 2 21 7 8 22 19 1 6 23 34 28 24 25 20