Essence of the sigma deviation method
The method of sigma deviations is one of the basic methods for assessing a child's physical development in child and adolescent hygiene. The essence of the method lies in comparing the child's individual anthropometric parameters (height, body weight, chest circumference) with the statistical mean values (M) for the corresponding sex and age, taking into account the permissible variation — sigma (σ), i.e., the standard deviation.
The method allows assessing two key parameters: the level of physical development and the proportionality of constitution. Additionally, harmoniousness is assessed when plotting the physical development profile.
Standard problem statement and required materials
A standard exam problem statement includes: the child's sex and age, body length (height), body weight, and chest circumference (CC). Example: a 7-year-old boy, height 120 cm, weight 25 kg, CC 63 cm. It is required to assess physical development using the sigma deviation method.
The solution requires:
- a non-programmable calculator;
- a standard table of age and sex standards indicating M and σ for each parameter.
Solution algorithm: constructing the calculation table
The calculation table contains five columns for each anthropometric parameter.
- Column 1 — actual value of the parameter from the problem statement (height 120 cm, weight 25 kg, CC 63 cm).
- Column 2 — mean value M, taken from the standards table at the intersection of sex, age, and the required parameter. For a 7-year-old boy, height M = 121.98 cm.
- Column 3 — sigma σ from the same table; for height, σ = 5.37 cm.
- Column 4 — difference between the actual and mean values: column 1 minus column 2. The sign must be preserved.
- Column 5 — sigma deviation: column 4 divided by σ (column 3). This is a dimensionless quantity showing by how many σ the individual parameter differs from the mean.
In the considered example for height, the deviation is about –0.36σ; deviations for body weight and CC are calculated similarly.
Assessment of the level of physical development
The level of development is assessed exclusively by the sigma deviation of body length (height), as height is the leading indicator of biological maturation. The sign of the deviation must be taken into account.
- from –1σ to +1σ — average level of development;
- from –1σ to –2σ — below average; from +1σ to +2σ — above average;
- less than –2σ — low level; more than +2σ — high level.
In the example, the height deviation of –0.36σ falls within the ±1σ interval — the level of physical development is average.
Assessment of development proportionality
Proportionality reflects the consistency between the main anthropometric features. To determine it, the maximum and minimum values in absolute value (ignoring the sign) are chosen from the sigma deviation column, and their difference is found.
- difference less than 1σ — proportional development;
- difference of 1σ or more — disproportional development.
In the example: |0.68| – |0.36| ≈ 0.32 — development is proportional.
Physical development profile and harmoniousness assessment
Physical development profile is a graphical representation of sigma deviations. A grid is constructed with vertical columns: –2σ, –1σ, M, +1σ, +2σ; three parameters are plotted horizontally: height, weight, CC. Points correspond to the calculated sigma deviations and are connected by a broken line.
Assessment of harmoniousness is performed only if a profile is available:
- points are located in one or two adjacent columns — harmonious development;
- points occupy three columns — disharmonious;
- points are distributed across four or more columns — sharply disharmonious.
If harmoniousness is not requested in the problem, plotting the profile is not required — this is a methodological feature of the sigma method.
Key points
- The sigma deviation method is based on comparing individual parameters with age- and sex-standardized mean values M ± σ.
- Calculation requires the child's actual data and a table of age and sex standards.
- Sigma deviation is calculated as (actual value – M) / σ while preserving the sign.
- The level of physical development is determined only by the height sigma deviation.
- Proportionality is assessed by the difference between the maximum and minimum deviations taken in absolute value.
- Harmoniousness is assessed exclusively from the constructed physical development profile.
- Critical boundaries: ±1σ — norm, ±2σ — boundary of developmental pathology.
- The method is applicable in the practice of a pediatrician and a child and adolescent hygiene physician.