Standard Deviation score (or z-score) is used to indicate how far a value is from the average value (or median) for a group. The average value is determined from a sample that represents the group. For example, to determine the average weight-for-height of boys, a representative sample of at least 200 healthy boys of a certain height (e.g.100cm) is identified and weighed. The average weight of this group represents the standard for boys of height 100cm and the SD score=0. This procedure is repeated for boys of 101cm etc.

If the values for a representative group are plotted, the result is a bell-shaped curve (see diagram). Most values are in the middle. Almost all are between -2SD and +2SD, and this is considered the normal range. If a value is below -3SD we can be very sure this is outside the normal range.

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