Z-scores can be used to compare a measurement to a reference value. The z-score is the number of standard deviations away from the average value of the reference group. This reference group usually consists of people of the same age and gender; sometimes race and weight are also included. The WHO fracture risk calculator uses Z-scores based on age and gender only; the Hologic machines give Z-scores for age, gender and race, and the Lunar machines for age, gender, race and weight.
This table shows how z-scores correspond to percentiles. A person who is average has a Z-score of zero and is at the 50th percentile. If 20% of people have a bone density below a person, he or she has a Z-score of -0.84 and is at the 20th percentile.
Pediatricians use percentiles to interpret the height of a child. A child at the 5th percentile (same as Z-score of -1.65) is short for his or her age, and one at the 75th percentile is somewhat taller than average (Z-score of 0.68). The Z-score does not tell how tall a child is, because the average child gets taller as she gets older. On the other hand, if you know that a child is 40" tall, it does not mean anything unless you also know his age. You must know both the age and the percentile to know if this is a healthy height.
Sunday, December 06, 2009
Z-scores
Z-scores can be used to compare a measurement to a reference value. The z-score is the number of standard deviations away from the average value of the reference group. This reference group usually consists of people of the same age and gender; sometimes race and weight are also included. The WHO fracture risk calculator uses Z-scores based on age and gender only; the Hologic machines give Z-scores for age, gender and race, and the Lunar machines for age, gender, race and weight.
This table shows how z-scores correspond to percentiles. A person who is average has a Z-score of zero and is at the 50th percentile. If 20% of people have a bone density below a person, he or she has a Z-score of -0.84 and is at the 20th percentile.
Pediatricians use percentiles to interpret the height of a child. A child at the 5th percentile (same as Z-score of -1.65) is short for his or her age, and one at the 75th percentile is somewhat taller than average (Z-score of 0.68). The Z-score does not tell how tall a child is, because the average child gets taller as she gets older. On the other hand, if you know that a child is 40" tall, it does not mean anything unless you also know his age. You must know both the age and the percentile to know if this is a healthy height.
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