may indicate a growth disorder. For example, a patient's bone age may be less than their chronological age suggesting a delay in growth as may be caused by a growth hormone deficiency. In the case of too much growth hormone, a child may have a bone age that is older than their chronological age suggesting that they are growing abnormally fast. Since bone age measurements are inherently approximations, they are conventionally reported with a standard deviation which serves as an estimate of the associated error. For a child's bone age to be considered abnormal, the chronological age must differ from the assigned bone age by more than 2 standard deviations. Bone age acts as a surrogate for physiological development because growth and maturation of the skeletal system depend on the presence of hormones like growth hormone, sex steroids (e.g., estrogen and testosterone), and thyroxine. Studies of bone age in children allow physicians to correlate a child's current height and bone age to their predicted future maximum height in adulthood. Not only can bone age help in diagnosing a child with a growth abnormality, but it can also play a role in treatment. In certain instances, abnormal growth conditions may be treated with supplemental hormone therapy. The best time to start and stop such therapies can be determined based on a patient's bone age. Height prediction Statistics have been compiled to indicate the percentage of height growth remaining at a given bone age. By simple arithmetic, a predicted adult height can be computed from a child's height and bone age. Separate tables are used for boys and girls because of the sex difference in timing of puberty, and slightly different percentages are used for children with unusually advanced or delayed bone maturation. These tables, the Bayley-Pinneau tables, are included as an appendix in the Greulich and Pyle atlas. In several conditions involving atypical growth, bone age height predictions are less accurate. For example, in children born small for gestational age who remain short after birth, bone age is a poor predictor of adult height. Evaluation of growth abnormalities For the average person with average puberty, the bone age would match the person's chronological age. In terms of height growth and height growth related to bone age, average females stop growing taller two years earlier than average males. Peak height velocity (PHV) occurs at the average age of 11 years for girls