The curves on the chart represent either centiles or fractions of an SD above or below the mean. The subject’s measurement is plotted on the chart and the corresponding centile or SD relative to the mean is to be noted. By convention, an individual’s position on the growth chart is known as the SD score (SDS) or z-score.
Comparison between the Centile and the SD score sea es
| Centile | SD score |
| Centiles are on a scale from 1 to 99 and centered on 50 | The SD score scale is centered on 0 |
| A centile refers to the percentage chance of a reference child having a smaller value than the subject under observation. | The SD score scale has an SD of 1, and is normally distributed |
| Centiles are easier for subjects and their parents to comprehend | SD scores are preferred by researchers, as they give better statistical results. |
There are two important points to remember; Firstly, the international growth reference uses SDs rather than centiles to quantify the size of malnourished individuals who lie well below the third centile and secondly, there is yet another method of using the percent of the median. Here the recorded anthropometric measurement is expressed as a percentage of the median value for the individual’s age and sex.
Percent of the median: This method is used mainly in the developing countries and is a simpler version of the SD score. The main difference between the two’ is that the former does not take into account the variability of the measurement, while the SD score does.
So we know that an individual’s position on the growth chart can be expressed as:
a centile
a SD score
a percentage of the median