Measuring bone density Individuals with low bone density have a high risk of broken bones (fractures). Physicians who are concerned about low bone density (osteoporosis) in patients can refer them for specialized testing.

Currently, the most common method for testing bone density is dual-energy X-ray absorptiometry (DEXA). A patient who undergoes a DEXA test usually gets bone density results in grams per square centimeter (g/cm2) and also in standardized units.

Francine, who is 25 years old, has her bone density measured using DEXA.

Her results indicate a bone density in the hip of 948 g/cm2, which converts to a standardized score of z 5 21.45. In the reference population of 25-year-old women like Francine,2 the mean bone density in the hip is 956 g/cm2.

(a) Francine has not taken a statistics class in a few years. Explain to her in simple language what the standardized score tells her about her bone density.

(b) Use the information provided to calculate the standard deviation of bone density in the reference population.