The data with regards to growth references can be either presented in a tabular form or as a chart plotted against age. The presentation in the form of charts has developed over the last hundred years (Tanner, 1981). The most significant underlying principle behind the preparation of the growth charts is that the curves should appear smooth.
The frequency distribution of each measurement is subsequently presented in several ways. At its simplest form, the mean and standard deviation (SD) ofthe measurement in question are tabulated by age and sex. Then, assuming a normal or Gaussian distribution, the mean and SD define the entire distribution and its centiles.
Now what is a centile? A centile is a point on the distribution that splits the population ..into two specified fractions. The median point is the 50th centile and is the midpoint of the distribution. It has 50% of the distribution to the left of it and 50% to the right. The third centile has 3% of the distribution to the left and 97% to the right.
Curves are drawn on the chart to represent the distribution at each age. Usually seven curves are drawn on the chart. The curves correspond to the mean, one SD above . and below the mean, two SDs above and below the mean and three SDs above and below the mean. As a result, each curve is placed one SD apart on the chart. The seven curve pattern has been used for the World Health Organization (WHO)/U.S. Centers for Disease Control (CDC)lNational Center for Health Statistics (NCHS) international growth references.
A set of several centiles can also be utilized in the growth chart to represent the range of the distribution. The usual set is the seven centiles 3’d, lO”,25th, 50th,75th, 90th, and 97th, approximately two thirds of an SO apart for a normal distribution.
The centiles are joined to form a centile curve. A centile curve, thus, is a curve joining the values of a specified centile at different ages. The percentage chance of an individual’s value lying below a given centile curve is shown by the value of the centile.