23- How to calculate standard deviation for height at a particular age?
The standard deviation (SD) for height at a particular age can be calculated from
a growth chart by subtracting the height at 50th percentile (cm) from the height
at 3rd percentile (cm) at that particular age and dividing it by 2. For example, in
CDC growth chart for boys at age 10 years, the height at 50th percentile is 138 cm
and the height at 3rd percentile is 127 cm. The difference between the two per-
centiles is 11 cm and dividing it by 2 gives a value of 5.5 cm, which is equivalent
to one SD at age 10 years in CDC growth chart for boys.- How to calculate standard deviation score for height at a particular age?
Standard deviation (SD) measures the deviation in height of a population from
the mean, whereas the standard deviation score (SDS) precisely quantifies the
height deficit of a child. The SDS is calculated as follows:(Heightof thechild−Heightat50pth ercentileforachildof sameage))÷SD
For example, if a boy has height of 115 cm at age of 10 years:- The mean height (50th percentile) at 10 years is 138 cm in CDC growth chart.
- The SD at this chronological age is calculated by subtracting height at 50th
centile from height at 3rd centile (138–127 = 11) divided by 2, i.e., 5.5 cm. - The SDS of the child is calculated as () 115 - 138 ̧ 55. , i.e., −4.2SDS.
The calculation of SDS in growth chart is illustrated below (Fig. 1.10).
1 Disorders of Growth and Development: Clinical Perspectives