hours per day, days per week, or weeks per year) spent with families or children. Unfortunately, however, relatively few effectiveness studies have collected sufficient data to assess this important variable. One important exception is the Infant Health and Development Program, which has generated a rich database on services received by individual sample members and has documented a positive association between intensity of participation and child cognitive gains (Ramey et al., 1992). The nonrandom nature of the differences in program participation, however, precludes definitive interpretation of intensity effects. In two studies of a home visiting program for poor families with infants in Jamaica, one of which used a random assignment design, weekly visits were associated with higher child developmental test scores than biweekly visits, and children who were visited bi-weekly scored higher than those who received services at monthly intervals (Powell and Grantham-McGregor, 1989). Intensity effects have also been noted for children with autism, as increased program intensity is associated with more substantial short- and long-term outcomes (McEachin et al., 1993; Lovaas, 1987).

Duration of intervention has also been studied and found to be associated with measurable family impacts. For example, mothers who participated in the Prenatal/Early Infancy Project for 2 years were less likely to maltreat their children than mothers who received 9 months of service. Moreover, a 15-year follow-up revealed an inverse relation between the amount of service received and a number of negative maternal outcomes, including child maltreatment, repeat pregnancy, welfare dependence, substance abuse, and interactions with the criminal justice system (Kitzman et al., 1997; Olds et al., 1997). The Abecedarian intervention plus a follow-up program into the elementary school years was more effective than the preschool intervention alone (Campbell and Ramey, 1994).

For families of young children with developmental disabilities, the variability in service intensity is considerable. Differences in both amount and duration of intervention may be related to the age of referral, the nature and severity of the child's impairment, or the family's resources and needs. In a systematic investigation of services received by families of children with Down syndrome, motor impairment, and developmental delays of uncertain etiology, the Early Intervention Collaborative Study documented an average of 7 hours of service per month over the first 12 months of program participation, with a range from less than 1 hour to over 20 hours monthly (Shonkoff et al., 1992) In this sample of 190 children enrolled in 29 community-based programs, the strongest predictor of service intensity in the first year was the child's score on the Bayley Scales at the time of program entry (i.e., children with more severe impairments received more hours of service).

The measurement of program intensity for children with disabilities

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