Table 2 Regression analyses. Factors associated with unknown smoking status. N = 855,704.

 Age of mother

1.02 (1.01–1.02)

1.00 (1.00–1.00)

 Parity

0.98 (0.98–0.99)

1.02 (1.02–1.03)

 Year of child birth

1.01 (1.01–1.01)

1.03 (1.03–1.03)

 Paper-based form

ref

ref

 Electronic form

0.47 (0.47–0.48)

0.58 (0.57–0.59)

 Norway/ Nordic Countries

ref

ref

 Europe outside the Nordic region

1.35 (1.32–1.38)

1.15 (1.12–1.17)

 Asia

1.68 (1.65–1.71)

1.17 (1.15–1.19)

 Africa

2.02 (1.98–2.07)

1.26 (1.23–1.29)

 Others

1.42 (1.35–1.49)

1.09 (1.04–1.14)

 No SGA-10

ref

ref

 SGA-10

1.15 (1.13–1.17)

1.02 (1.00–1.04)

 Married, living together, etc.

ref

ref

 Divorced, living alone, etc.

1.01 (1.00–1.03)

1.02 (1.00–1.04)

 Medium education

ref

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 University education

1.19 (1.18–1.21)

0.97 (0.96–0.98)

 Primary education

1.13 (1.12–1.15)

1.04 (1.02–1.06)