The weight Z-score is a useful concept when studying the nutrition and growth of individuals. The weight Z-score is the number of standard deviations by which a weight differs from the mean at a specific age. A Z-score of-1 is a weight on the -1 SD line (=46th centile). A AZ-score indicates the change in the Z-score over a time interval. A negative AZ-score accompanies an individual who is 'falling off their centiles' even if gaining weight slowly. The Z-score can be estimated from a centile chart, calculated from the mean and SD or looked up in a table. If the population data are skewed, then complex modelling can been applied, as seen in the WHO charts, but the arguments outlined here still stand. Observant researchers will have noticed this asymmetry in that the +1 and -1 SD lines may not be symmetric about the mean, and they may also have noticed that many term weight charts have no centile lines for the first few days of chronological age.
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