Charts to Measure Size

A growth chart summarizes the distribution of the measurement (e.g., weight) as it changes with age in some prespecified reference population (e.g., British children measured in 1990). At its simplest, the chart consists of the median curve, a smooth curve connecting median weight for the population at different ages. Usually, however, there are curves for other distribution centiles as well,

extending typically from the 3rd to the 97th cen-tile, to give an idea of the spread of measurements at each age. A centile corresponds to a given percentage (between 0 and 100) and is a measurement below which that percentage of children in the reference population will be found. For example, in the British 1990 boys weight reference, median (or 50th centile) weight at lyear is 10.1kg, whereas the 3rd and 97th centiles are respectively 8.3 and 12.3 kg. Therefore, 50% of British boys aged 1year weigh less than 10.1kg, whereas 3% weigh less than 8.3 kg and 97% weigh less (and 3% weigh more) than 12.3kg.

The lowest centile curve on the chart is often used as a cutoff to detect poor growth. Children with measurements falling below this centile are viewed as 'at risk' and may be referred for more detailed examination. Figure 1 shows the British weight reference for boys in infancy, which has nine centile curves ranging from the 0.4th at the bottom to the 99.6th centile at the top. The value of these more extreme centiles for screening is explained later.

Using a centile curve as the cutoff on the chart is just one approach to identifying at-risk children. In the developing world, where the concept of centiles can be difficult to explain, a simpler alternative is 'percent of the median.' Here, the cutoff is constructed as a curve that is, for example, 80% of the median weight curve, which is broadly similar in shape to the 3rd centile curve. A child whose weight falls below the cutoff is said to be below 80% weight for age.

Another approach to defining cutoffs involves standard deviation (SD) scores, also called z scores. These are linked to centiles through the underlying frequency distribution, in particular the SD of the measurement at each age. With a normal distribution, the median and mean coincide and the SD

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