5 . Measures of relative standing (percentiles and quartiles), interquartile range

Agrowth chartis a plot of the percentiles of growth measurements, such as weight and height, for a population of infants or children. It is used by pediatricians to assess a child's growth over time.

The Centers for Disease Control and Prevention (CDC) is a U.S. agency devoted to the protection and promotion of public health. Through one of its units, the National Center for Health Statistics (NCHS), the CDC has developed growth charts for clinical use by health professionals. The most recent charts were published in 2000.

The 2000 CDC growth charts were developed using a reference population of infants. A pediatrician looks up one of the charts and finds that the 90th percentile for weights of baby boys at 15-1/2 months is 26.4 pounds. This means that of the 15-1/2-month-old baby boys in the reference population weigh 26.4 pounds or less, and of these baby boys weigh 26.4 pounds or more.

The 2000 CDC growth charts use a reference population of both breast-fed and formula-fed infants. It has been observed that breast-fed babies tend to gain weight more rapidly than formula-fed babies in the first 2 to 3 months of their lives, but they tend to weigh less than formula-fed babies from 6 to 12 months.

Sarah is a healthy baby who was exclusively breast-fed for her first 12 months. Which of the following is most likely a description of her weights (at 3, 6, 9, and 12 months of age) as percentiles of the CDC growth chart reference population?

30th percentile at 3 months; 50th percentile at 6 months; 70th percentile at 9 months; 80th percentile at 12 months

85th percentile at 3 months; 85th percentile at 6 months; 90th percentile at 9 months; 95th percentile at 12 months

75th percentile at 3 months; 40th percentile at 6 months; 25th percentile at 9 months; 25th percentile at 12 months

25th percentile at 3 months; 25th percentile at 6 months; 25th percentile at 9 months; 25th percentile at 12 months

The following are weights (in pounds) for a sample of 11 baby girls at the age of 21 months:

21.922.223.124.425.726.327.528.629.831.533.1

The 85th percentile for these sample data is pounds.

In the sample data, out of 11 observations are less than or equal to the 85th percentile, and out of 11 observations are greater than or equal to this value.

For any data set, the first quartile is the percentile, the second quartile is the percentile and the , and the third quartile is the percentile.

The first quartile for the prior sample data is pounds.