Since children’s height and weight are dependent on age and sex, height and weight measurements must be standardised to take these factors into account. The standardised value is called a z-score and indicates how far, and in what direction, the measurement deviates from the average (mean) for that age and sex.
High and low z-scores (i.e. measurements that are significantly higher or lower than the mean) are less likely to occur and indicate extreme values.
The NCMP system flags measurements as being ‘extreme’ if the z-score is less than -3 or more than 4.
To calculate the z-score:
- Look up child age and sex on the relevant UK90 centiles classification (there is a separate classification for height, weight and BMI)
- Retrieve the corresponding L, M, and S values for use in the following formula (where y is the measurement for example. height, weight or BMI):
There are LMS values for each whole month. Where age falls between two whole months, linear interpolation is used to estimate LMS values. This involves assuming a linear relationship between age and LMS values and deriving the LMS values based on where the age sits between the two months. More information is available at Freeman JV, Cole TJ, Chinn S, Jones PRM, White EM, Preece MA. Cross sectional stature and weight reference curves for the UK, 1990. Archives of Disease in Childhood 1995;73: 17‐24