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Anthropometric Standardization (Growth Charts)

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This function provides standard deviation scores (z scores) for common anthropometric measures in children, such as height, weight, and BMI, with reference to British, US and WHO growth standards.

The method of standardisation is LMS (lambda, mu, sigma) of Cole et al. (1992) with extensions (World Health Organisation 2006, 2007; Vidmar et al. 2013).

You provide:

• the measure (e.g. height) that needs standardising to make it comparable across children of different ages^*;
• the age^* of the child (optionally with gestational age for some growth charts to adjust for the 'catch-up growth' of children born before 40 weeks);
• the sex of the child; and
• a reference standard appropriate for your study population (the available reference standards are described below).

[^* weight can be standardised for length or height as well as age: see reference standards below]

StatsDirect returns the z score for the measure, which is the number of standard deviations the child's measurement is from the average child of equivalent age (+ gestational age) and sex in the reference population.

The standard deviation score z ranges from -5 to +5 because StatsDirect considers more extreme values to be erroneous, so returns them as missing values.

In terms of percentiles: z = 0 is the 50th percentile, or exactly average; z = 1 would be the 84th percentile and z = -1 would be the 16th percentile. The percentile is calculated using the standard normal distribution function (lower tail), which is the proportion of normal children expected to have that measurement or a lower measurement.

For BMI, in addition to a z score, StatsDirect returns a category [G3 Thin = grade 3 thinness; G2 Thin = grade 2 thinness; G1 Thin = grade 1 thinness; Normal = normal weight; Overweight = overweight; Obese = obese]. The adult equivalent BMI cut-off levels for these categories are: <16 = G3 Thin; 16 to <17 = G2 Thin; 17 to <18.5 = G1 Thin; 18.5 to <25 = Normal; 25 to <30 = Overweight; >30 = Obese.

The growth reference standards included are:

British Growth Reference 1990: UK

Height/length (0-23 years)^*

Weight (0-23 years)^**

BMI (0-23 years)^*

Head circumference (0-18 years for males, 0-17 years for females)^**

Sitting height (0-23 years)

Leg length (0-23 years)

Waist circumference (3-17 years)

Body fat percentage (4.75-19.83 years)

[^* can be adjusted for gestational age from 33 weeks; or ^** 23 weeks]

Centres for Disease Control (CDC) 2000: US

Length (0-35.5 months)

Height (2-20 years)

Weight (0-20 years) or (45-103.5 cm length) or (77-121.5 cm height)

BMI (2-20 years)

Head circumference (0-36 months)

World Health Organisation (WHO) 2007: Term gestation

Height/length (0-19 years)

Weight (0-10 years) or (45-110 cm length) or 65-120 cm height)

BMI (0-19 years)

Head circumference (0-5 years)

Arm circumference (0.25-5 years)

Subscapular skin fold (0.25-5 years)

Triceps skin fold (0.25-5 years)

WHO 2007: Pre-term

Height/length (0-20 years)^*

Weight (0-20 years)^**

BMI (0.038-20 years)

Head circumference (0-18 years for males, 0-17 years for females)^**

[^* can be adjusted for gestational age from 25 weeks; or ^** 23 weeks]

WHO 2009: Term

Height/length (0-20 years)^*

Weight (0-20 years)^*

BMI (0.038-20 years)

Head circumference (0-18 years for males, 0-17 years for females)^*

[^* can be adjusted for gestational age from 37 weeks]

Example

Test workbook (Other worksheet: BMI, Age (Years), Sex).

Selecting the British 1990 Growth Reference standard for the above gives:

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