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Table 2 Standardised Differences across Covariates: before Matching and after Matching

From: Association between intimate partner violence during pregnancy and risk of preterm birth

  Before matching After matching
IPV No IPV p-value %bias IPV No IPV p-value %bias
Age (Years)
 20–24 14.6 22.8 0.05 −21.1 14.6 17.3 0.596 −7
 25–29 24.3 26.4 0.635 −4.8 24.3 25.0 0.898 −1.8
 30–34 24.3 24.1 0.968 0.4 24.3 21.2 0.597 7.2
 35–39 20.4 13.4 0.041 18.7 20.4 20.8 0.945 −1
 40–44 6.8 6.4 0.884 1.4 6.8 6.2 0.866 2.3
 45–49 2.9 1.2 0.135 11.8 2.9 2.5 0.865 2.7
Level of Education
 Primary 26.2 28.7 0.578 −5.6 26.2 20.6 0.342 12.6
 Secondary plus 73.8 71.3 0.578 5.6 73.8 79.4 0.342 −12.6
Husbands’ level of education
 Primary 17.5 19.8 0.565 −5.9 17.5 16.3 0.824 3
 Secondary plus 81.6 79.1 0.546 6.1 81.6 83.1 0.771 −3.9
Wealth Index
 Middle 11.7 15.4 0.297 −11 11.7 14.0 0.619 −6.8
 Rich 63.1 47.5 0.002 31.7 63.1 62.7 0.954 0.8
Work status
 Not working 51.5 49.3 0.67 4.2 51.5 53.0 0.824 −3.1
Residence
 Urban 43.7 61.6 0 −36.4 43.7 41.7 0.779 3.9
Smoke
 Cigarette smoking 1.0 0.1 0.032 11.3 1.0 1.2 0.893 −2.6
Age at first intercourse
 15–17 33.0 39.9 0.156 −14.4 33.0 30.7 0.721 4.8
 18+ 63.1 54.8 0.093 17 63.1 66.4 0.622 −6.7
Prenatal Care 93.2 94.5 0.577 −5.3 93.2 93.8 0.866 −2.4
Number of antenatal visits
 4–7 times 34.0 48.4 0.004 −29.5 34.0 36.5 0.706 −5.2
 8–20 times 16.5 12.5 0.228 11.3 16.5 14.8 0.731 5
Wanted pregnancy 62.1 68.9 0.143 −14.2 62.1 61.9 0.977 0.4
  1. *The standardised percentage bias. The standardised % bias is the % difference of the sample means in the treatment and control (full or matched) sub-samples as a percentage of the square root of the average of the sample variances in the treated and non-treated groups [30]