Selection criteria | 1994 Birth cohort (ascertained as having ASD in 2002 ADDM) | 2000 Birth cohort (ascertained as having ASD in 2008 ADDM) | ||||
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No. of sites | No. of counties | No. of cases | No. of sites | No. of counties | No. of cases | |
Selection of final study population (population to which results can be generalized) | ||||||
ADDM sites included in both 2002 and 2008 surveillance years | 13 | 175 | 2529 | 13 | 98 | 3609 |
Limit to: counties included in both 2002 and 2008* | 13 | 94 | 1825 | 13 | 94 | 3285 |
Limit to: children residing in same county at birth and age 8† | 10 | 69 | 951 | 10 | 69 | 1880 |
Limit to: ADDM site that provided all necessary study variables | 8 | 48 | 786 | 8 | 48 | 1551 |
Limit to: singletons | 8 | 48 | 762 | 8 | 48 | 1489 |
Limit to: race-ethnicity NHW, NHB, Hispanic | 8‡ | 48 | 747 | 8 | 48 | 1406 |
Sample exclusions | ||||||
Children excluded if they lacked data on any study variable | 726 | 1386 | ||||
Children in matching stratum with <20 controls/case excluded§ | 703 | 1339 |
Not all ADDM sites included the same counties in their surveillance area in 2002 and 2008.
In addition to limiting to children identified as ASD cases in ADDM who were linked to a birth record indicating birth residence was within the ADDM catchment area, this restriction also pragmatically limited our sample to ADDM sites that included the maternal county of residence indicator in their submitted ADDM-natality data set; three sites did not.
Final study population included NHW, NHB, and Hispanic children born as singletons in 48 counties within eight ADDM sites.
Twenty controls per case were selected using a matched design in which cases and controls were matched on five sociodemographic factors. Thus, for control selection, the study population was subdivided into numerous matching strata and some included a small number of births. The final sample selection criterion was birth within a study-matching stratum including a minimum of 20 potential controls.