The National Center for Health Statistics 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 below in the table:
(Data is taken from pg. 260, Elementary Statistics, Larson and Farber, 2015)
Gestation period | Mean birth weight | Standard deviation
Under 28 weeks | 1.90 lb | 1.23 lb
28 to 31 weeks | 4.10 lb | 1.88 lb
32 to 33 weeks | 5.08 lb | 1.56 lb
34 to 36 weeks | 6.14 lb | 1.29 lb
37 to 38 weeks | 7.06 lb | 1.09 lb
39 weeks | 7.48 lb | 1.02 lb
40 to 41 weeks | 7.67 lb | 1.03 lb
42 weeks and over | 7.56 lb | 1.10 lb
1) What percent of babies born within each gestation period have a low birth weight (under 5.5 pounds)?
a) 28 to 31 weeks
b) 40 to 41 weeks
2) Describe the weights in the 90th percentile of the babies born within each gestation period.
a) 34 to 36 weeks
b) 39 weeks
3) For each gestation period, what is the probability that a baby will weigh between 6 and 9 pounds at birth?
a) 42 weeks and over
b) 37 to 38 weeks
4) A birth weight of less than 3.25 pound 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 each gestational period?
a) 32 to 33 weeks
b) Under 28 weeks