P50 = mean = mode = median = 2nd quantile
Mean + / – 1 × SD = 68.27 % :
between 16th and 84th percentile
Mean + / – 2 × SD = 95.45 % :
between 2nd and 98th percentile
Mean + / – 3 × SD = 99.73 % :
between 0.1th and 99.9 th percentile
Z-score = (height – mean height) / standard deviation
The Z-score is the number of standard deviations the
bone mineral density measurement is above or below
the age-matched mean bone mineral density.
Z-score = (BMD – age-matched mean BMD) / standard deviation
(BMD = bone mineral density)
The T-score is the number of standard deviations the
bone mineral density measurement is above or below
the young-normal mean bone mineral density.
T-score = (BMD – young-normal mean BMD) / standard deviation
(BMD = bone mineral density)
World Health Organisation definition (1994):
Normal: T-score -1 or above
Osteopenia: T-score between -1 and -2.5
Osteoporosis: T-score less than -2.5
Severe osteoporosis: T-score less than -2.5 and fragility fracture