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Table 2 Potential confounding factors in relation to case-control status

From: Prenatal smoking exposure and neuropsychiatric comorbidity of ADHD: a finnish nationwide population-based cohort study

   Cases Controls p χ 2
Maternal age at birth ≤19 years 672 (6.7 %) 899 (2.4 %) p < 0.001 603.69
20–29 years 5711 (56.5 %) 18,861 (51.1 %)
30–39 years 3475 (34.4 %) 16,143 (43.7 %)
≥40 years 254 (2.5 %) 1020 (2.8 %)
Paternal age at birtha ≤19 years 211 (2.2 %) 212 (0.6 %) p < 0.001 330.41
20–29 years 4322 (44.2 %) 13,966 (38.2 %)
30–39 years 4358 (44.5 %) 18,892 (51.7 %)
≥40 years 896 (9.2 %) 3496 (9.6 %)
History of maternal psychopathology Yes 2610 (25.8 %) 4340 (11.8 %) p < 0.001 1154.75
No 7502 (74.2 %) 32,583 (88.3 %)
History of maternal substance abuse Yes 685 (6.8 %) 747 (2.0 %) p < 0.001 525.72
No 9427 (93.2 %) 36,176 (98.0 %)
History of paternal psychopathologya Yes 2382 (24.3 %) 4597 (12.6 %) p < 0.001 779.19
No 7405 (75.7 %) 31,969 (87.4 %)
Previous births 0 4683 (46.3 %) 15,077 (40.8 %) p < 0.001 98.03
≥1 5429 (53.7 %) 21,846 (59.2 %)
Weight for gestational ageb <-2 SD 569 (5.6 %) 1017 (2.8 %) p < 0.001 189.09
-2 to +2 SD 9167 (91.0 %) 34,542 (93.8 %)
≥ + 2 SD 338 (3.4 %) 1257 (3.4 %)
Maternal SES Upper white collar 861 (8.5 %) 5628 (17.7 %) p < 0.001 593.56
Lower white collar 3986 (39.4 %) 6522 (15.2 %)
Blue collar 2439 (24.1 %) 16,528 (44.8 %)
Others 1959 (19.4 %) 5693 (15.4 %)
Missing 867 (8.6 %) 2552 (6.9 %)
Maternal immigrant status Not born in Finland 223 (2.2 %) 542 (1.5 %) p < 0.001 26.04
Born in Finland 9889 (97.8 %) 36,381 (98.5 %)
Paternal immigrant statusa Not born in Finland 368 (3.8 %) 651 (1.8 %) p < 0.001 136.41
Born in Finland 9419 (96.2 %) 35,915 (98.2 %)
Apgar score at 1 minc <6 537 (5.3 %) 1376 (3.7 %) p < 0.001 48.65
7–10 9565 (94.7 %) 35,504 (96.3 %)
Gestational age 32 weeks 224 (2.2 %) 245 (0.7 %) p < 0.001 219.24
3336 weeks 564 (5.6 %) 1408 (3.8 %)
3741 weeks 8833 (87.6 %) 33,506 (91.0 %)
42 weeks 460 (4.6 %) 1677 (4.5 %)
  1. adata missing 682 subjects
  2. bdata missing: 107 subjects
  3. cdata missing 53 subjects