Example 1.4.3: Case study: CDC data on births in the United States. A study published by the U.S. Centers for Disease Control on the 2016 birth data presents several health indicators such as fertility rates, birth rates, and multiple birth rates. The chart below appears in a section titled "The preterm birth rate rose for the second straight year in 2016." Preterm birth refers to a pregnancy that is shorter than normal, especially births that occur before the 37th week of pregnancy. The chart displays the percentage of preterm births for years 2014, 2015, and 2016 categorized by the mother's age group. The age groups are: under 20, 20-29, 30-39, and 40 and over. A vertical stacked bar chart showing the percentage late and early preterm births for different age groups for years 2014, 2015, and 2016 for all ages, under 20, 20-29, 30-39, and 40 and over. Ex: In 2016, for mothers under 20, the percentage of early preterm births is 3.20 and the percentage of late preterm births is 7.20. In 2015, the percentage of preterm births is 9.63. In 2016, the percentage of preterm births is 9.85. In 2016, percentages of late preterm births for under 20