The SUPERB sample consisted of 499 northern California families and 156 central California residents, with the original enrollment goal set at 500 families with young children and 250 older adults enrolled in Tier 1 (see Table ). Enrollment in Tier II was conducted by recruiting from among eligible participants in Tier I. The target was to enroll a total of 250 households and the final number enrolled was 186 northern California and 64 central California households. For Tier III, we also recruited from Tier I participants who were not already participating in Tier II; our goal was a total of 40 households. We exceeded our goal, with the final tally of 47, of which 30 households were from northern, and 17 from central California.
Enrollment and completion rates of Tiers I, II, and III
The enrollment rate for Tier I was 25.5%, enrolling 499 out of 1,955 families contacted (Figure ). The most common reason given for refusal was lack of interest (n = 462) followed by lack of time (n = 217), passive refusal (never said no, but never said yes, n = 64), an unstated reason (n = 37), and lack of trust (n = 19). In central California, of 527 older individuals reached by mail (n = 132), or both mail and phone (n = 395), 167 (31.7%) enrolled in Tier I of the study (this figure does not include the non-random home visits during which an additional 18 consented and an unknown number of those actually enrolled). There were higher enrollment rates were for Tier II at 77% and 75% for northern and central California, respectively, compared to enrollment rates for Tiers III of 32% and 52%, both of which were recruited from Tier I. However, Tier II also had the lowest retention rates, with 64% and 28% of northern and central California, respectively, not fully completing the survey instruments compared to 3-12% for the other Tiers. The first year of Tier I surveys, during which we conducted 5 surveys over approximately one month, had the highest overall completion rate (87% and 95% for northern and central CA) but these rates dropped in the longitudinal data collection in which the full completion rate (all 3 years of surveys) was 38% and 41% respectively. In Tiers II and III, completion rates were mixed. As mentioned above, Tier II had the lowest full completion rates compared to other Tiers but rates differed between cohorts with older adults having a much higher full data completion rate (63% vs. 29%). Tier III had full data completion rates (4 visits over 16 months), of 90% and 76% for northern and central California participants, respectively, nearly as high as Tier I first year data collection.
Table provides a description of contaminants we considered when deriving relevant questions for the survey instruments. Thirteen categories of compounds were targeted for investigation based on their widespread use and high likelihood of being a source of contamination in the home environment. Data on exposure to these compounds was collected via seven categories including: residential information, food frequency, 24-hour food recall, supplemental food questions, temporal-spatial activity questions, consumer products, and personal care products. For example, pesticide information came from the food frequencies and 24-hour food recalls (fruits and vegetables high in pesticide residues), household product use (sprays or foggers used indoors or outdoors), and personal care product use (insect repellant).
Description of contaminants in the home environment included in the study
Table shows demographic characteristics collected on enrolled subjects. From both northern and central California, the adult respondents were primarily females (data from Tier I interviews): >80% females in Tiers I and II from households with young children, and about two-thirds from households of persons over 55 years of age. The Tier I sample was 47% Latino or nonwhite in the northern California households of families with young children, and 24% Latino or nonwhite in the central California sample of older adults. In Tier II, these figures were 34% and 14% in northern and central California, respectively, and in Tier III, 23% and 31%. Thus, the trend was reversed, such that nonwhite families were progressively less likely to participate when moving from Tier I to II to III in the Northern California cohort, whereas diversity of older adults was highest in Tier III. In northern California, employed persons comprised around 50% of participants in Tiers I and II, this dropped to 35% in Tier III; among older adults in central California, the figures for employed persons were 40% in Tiers I and II and this dropped to 25% for Tier III. In both age groups/regions, the percentage married was higher for Tier II than for Tier I or III. Place of birth (U.S. vs. abroad) did not differ dramatically across the Tiers. In the first year of Tier I data collection, 10.2% of northern California participants took the survey in Spanish, as compared with 2.6% of the central California older adults. Similar to the northern California cohort, the central California sample represented a relatively privileged group- the majority being home owners (86.6%) compared to a home ownership rate of 53.8% among the general population of the (combined data from included central California counties, data not shown) [22
]. Among minors enrolled (n = 566, we sometimes enrolled two per household), the age distribution reflected the emphasis we placed on the early life stage primarily recruiting from among early life stages: early childhood (<6 years): n = 468 (82.7%), middle childhood: (6-12 years) n = 77 (13.6%) and adolescence (12- 17 years): n = 21 (3.7%) (data not shown).
Demographic characteristics, as reported by respondent during interview, for the northern and central California cohorts
In comparison with the target population for Tier I northern California families, those who participated in the study were significantly older, more educated, and less likely to have had a publicly funded delivery (Table ).
Maternal demographic characteristics recorded on birth certificates of northern California samplesa
The majority of SUPERB participants possessed a computer and an internet connection at the start of the study. Among Tier I families (n = 499), 84.0% (n = 419) owned a computer and 79.8% (n = 398) had internet service. Among the older adults in the study sample (n = 156), 78.9% (n = 123) had a computer and 73.7% (n = 115) had internet service (data not shown).