Health care professionals sometimes convert height, weight or body mass index (BMI) measurements to standard deviation scores (SDS). The standard deviation is a measure of the amount of variation or spread of a set of values around the mean or average (also called the expected value). The mean or average value is given an SDS of 0. A lower standard deviation score means that the measurement is closer to the average or mean, while a high standard deviation score means that the value is further from the average or mean. A negative SDS indicates that the value is below the average or mean and a positive value means it is above the average or mean.
Growth charts allow us to understand whether a measurement is normal by comparing it with the normal range of measurements for other children of the same age and sex. Most growth charts show percentile (or centile) lines. The percentile number (or centile) shows how a child’s height compares to children of the same age and sex. The middle of the ‘normal’ range is the 50% percentile. For example, if your child is on the 75th percentile for height, they are taller than 75% of children of the same age and sex and 25% of children are taller than him/her. The 10th percentile means 10% of children of the same age and sex are shorter or 90% of children are taller.
To see how and SDS score compares to the centiles on the UK-WHO growth charts see the table below:
Standard Deviation Score (SDS)
Equivalent growth chart percentile