Charts relating height to age have been devised, and give an indication of the normal rate of growth, particularly in childhood. The wide variation in normal children is immediately apparent on studying such charts. Centile or percentile charts describe the distribution of a characteristic in a population. They are obtained by measuring a specific characteristic in a large population of each sex at each age. For each age there will be a height, above and below which 50 per cent of the population lies: this is called the 50th centile. The 50th centile thus indicates the mean height at a particular age. Such tables are less reliable around the age of puberty, because of variation in age of onset.