The March of Dimes disparity ratio is based on Healthy People 2020 methodology and provides a measure of the differences, or disparities, in preterm birth rates across racial/ethnic groups within a geographic area.1 The disparity ratio compares the racial/ethnic group with the lowest preterm birth rate (comparison group) to the average of the preterm birth rate for all other groups.
To calculate the disparity ratio, the 2013-2015 preterm birth rates for all groups (excluding the comparison group) were averaged and divided by the 2013-2015 comparison group preterm birth rate. The comparison group is the racial/ethnic group with the lowest six-year aggregate preterm birth rate (2010-2015) among groups that had 20 or more preterm births in all years from 2010-2015. A disparity ratio was calculated for each U.S. state (excluding Maine, Puerto Rico, Vermont, and West Virginia), the District of Columbia, and the total U.S. A lower disparity ratio is better, with a disparity ratio of 1 indicating no disparity. Progress towards eliminating racial and ethnic disparities was evaluated by comparing the 2013-2015 disparity ratio to a baseline (2010-2012) disparity ratio. Change between time periods was assessed for statistical significance at the 0.05 level using the approach recommended by Healthy People 2020.1
If the disparity ratio significantly improved because the average preterm birth rate for all other groups got better, we displayed "Improved" on the report card. If the disparity ratio significantly worsened because the lowest group got better or the average of all other groups got worse, we displayed "Worsened" on the report card. If the disparity ratio did not significantly change, we displayed "No Improvement" on the report card.
The report card also provides the percent difference between the racial/ethnic group with the 2013-2015 highest preterm birth rate compared to the combined 2013-2015 preterm birth rate among women in all other racial/ethnic groups. This percent difference was calculated using only the racial/ethnic groups displayed on the state or jurisdiction-specific report card. This difference was calculated for each U.S. state with adequate numbers and the District of Columbia.
1Talih M, Huang DT. Measuring progress toward target attainment and the elimination of health disparities in Healthy People 2020. Healthy People Statistical Notes, no 27. Hyattsville, MD: National Center for Health Statistics. 2016.