"Thirst strike": hypernatraemia and acute prerenal failure in a prisoner who refused to drink.

observed 12 month height velocity at the mean value for the population-'that is, 6 27 cm a year on standard charts-the 95% confidence interval for his true velocity ranges from the 25th centile to the 75th. However, if another boy aged 2 years also has an observed velocity on the 50th centile-8-31 cm a year in this case-the 95% confidence interval ranges from the 21st centile to the 79th. This degree of imprecision in determining the rate of growth, at any age, means that in practice the calculation of a single velocity, even over 12 months, is of limited clinical use.3 The measurement of height must therefore begin at the earliest possible age and be repeated at frequent -intervals to observe the trend, ideally by the same observer. The data for each child measured should be plotted on an individual height chart, and an explanation sought for unexpected changes in centile.