The National Center for Health Statistics (NCHS) keeps records of many health-related aspects of people, including the birth weights of all babies born in the United States. The birth weight of a baby is related to its gestation period (the time between conception and birth). For a given gestation period, the birth weights can be approximated by a normal distribution. The means and standard deviations of the birth weights for various gestation periods are shown in the table below. One of the many goals of the NCHS is to reduce the percentage of babies born with low from 2007 to 2015. birth weights. The figure below shows the percents of preterm births and low birth weights Gestation Mean birth Standard period weight deviation 12 Preterm = under 37 weeks Under 28 weeks 1.60 lb D.76 1b wow birth weight = under 5.5 pounds 28 to 31 weeks 320 1b 1.02 1b 32 to 33 weeks 4.31 1b 0.97 1b Percent of preterm births 34 to 36 weeks 5.74 1b 1.13 1b 37 to 38 weeks 6.92 1b 1.07 1b Percent of low birth weights 39 to 40 weeks 7.60 1b 0.99 1b 41 weeks 8.00 1b 0.99 1b 2107 20106 2309 2110 2011 2012 2713 2014 215 42 weeks and over 8.16 1b 1.12 1b Year EXERCISES 1. The distributions of birth weights for three 2. What percent of the babies born within each gestation periods are shown. Match cach curve gestation period have a low birth weight (under with a gestation period. Explain your reasoning. 5.5 pounds)? (a) (a) 28 to 31 weeks (b) 32 to 33 weeks (c) 39 to 40 weeks (d) 42 weeks and over 3. Describe the weights of the top 10% of the babies born within each gestation period. Pounds (a) Under 28 weeks (b) 34 to 36 weeks (b) (c) 41 weeks (d) 42 weeks and over 4. For each gestation period, what is the probability that a baby will weigh between 6 and 9 pounds at birth? (a) Under 28 weeks (b) 32 to 33 weeks Pounds (c) 37 to 38 weeks (d) 41 weeks (c) 5. A birth weight of less than 3.25 pounds is classified by the NCHS as a "very low birth weight." What is the probability that a baby has a very low birth weight for cach gestation period? (a) Under 28 weeks (b) 28 to 31 weeks Founds (c) 32 to 33 weeks (d) 39 to 40 weeks 260 CHAPTER 5 Normal Probability Distributions