Table 2: Chi-square analyses testing for racial differences in PTB rates stratified by maternal age and income.

Full sample (6,472)African-American (781) White (691)Chi-squarea

Overall PTB rate9.6%10.7%8.7%19.55

Low (<$31,389)10.2%10.4%9.5%0.77
Middle ($31,389–$48,466) 9.5%10.1%9.1%2.56
High (>$48,466)9.4%12.5%7.5%28.16

Age <17 years10.5%12.2%8.2%8.13
Age 17–199.5%10.5%8.7%12.55

Low income, age <17 years10.9%11.5%8.9%0.75
Low income, age 17–1910.0%10.2%9.6%0.32
Middle income, age <17 years10.5%12.3%8.7%3.02
Middle income, age 17–199.4%9.8%9.1%0.33
High income, age <17 years9.9%13.5%7.0%5.53
High income, age 17–199.3%12.4%7.6%22.71

Significant differences between racial categories, , , .
Chi-square statistics reflect tests for racial differences only. Other analyses for the full sample (not shown) found no significant differences between younger and older mothers ( = , ) or between mothers in each of the three income groups (, ).