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Short-term and long-term variability of standard deviation scores for size in children
1Abteilung Biostatistik, ISPM, University of Zurich, Zurich, Switzerland.
Insights
Girls exhibit greater long-term variability in skeletal growth standard deviation scores (SDS) than boys. This study analyzed growth patterns in children, revealing differences in variability across skeletal measurements and body mass index (BMI).
Area of Science:
- Human growth and development
- Pediatric endocrinology
- Biostatistics
Background:
- Understanding variability in children's growth is crucial for identifying normal development and potential deviations.
- Longitudinal growth studies provide valuable data for analyzing growth patterns over time.
- Distinguishing between long-term and short-term variability in growth metrics is essential for accurate interpretation.
Purpose of the Study:
- To quantify long-term and short-term variability in standard deviation scores (SDS) for skeletal size and body mass index (BMI) in children.
- To compare growth variability between boys and girls.
- To compare variability across different skeletal variables and BMI.
Main Methods:
- Analysis of longitudinal data from 120 boys and 112 girls aged 1 month to 20 years.
- Measurement of seven variables: standing height, sitting height, leg height, arm length, biiliac width, bihumeral width, and BMI.
- Separation of variability due to pubertal spurt timing by using developmental age instead of chronological age.
- Modeling SDS-age relationship to define long-term (smooth function departures) and short-term (error term variance) variability.
Main Results:
- Girls showed significantly higher long-term variability in SDS compared to boys; no significant sex difference in short-term variability.
- Bihumeral width, BMI, and sitting height exhibited significantly greater long-term variation.
- Bihumeral width and BMI had the highest short-term variability, while standing height had the least.
- Correlations between long-term variability and adult size, pubertal spurt timing, and intensity were low.
Conclusions:
- A robust method for assessing long-term and short-term variability in SDS has been developed and applied.
- Girls' growth processes demonstrate greater long-term variability than boys', a finding that warrants further investigation.
- Observed differences in variability across skeletal parameters are noteworthy and require additional study.
Primary Objective:
To quantify long-term and short-term variability in the standard deviation scores (SDS's) for six skeletal size variables and body mass index (BMI) in children and to compare average values of these quantities for boys with those of girls and to make comparisons across variables.
Methods And Procedures:
The analysis is based on measurements made regularly for 120 boys and 112 girls from 1 month until 20 years for seven variables (standing height, sitting height, leg height, arm length, biiliac width, bihumeral width and BMI) as part of the first Zurich longitudinal growth study. Variation in these scores due to variablity in the timing of the pubertal spurt (PS) is separated out by rescaling the age axis on an individual basis and comparing children with the same developmental age rather than the same chronological age. For a given child, the relationship between the value of its SDS and age is modelled as the sum of an arbitrary (child dependent) smooth function plus an error term. The long-term variability for that child is defined to be the mean square of the departures of this smooth function from its mean level while the short-term variability is defined to be the variance of the error term.
Main Outcomes And Results:
Girls' SDS scores have significantly more long-term variability than those of boys, while there is no significant difference between the sexes for short-term variability. Bihumeral width, BMI and sitting height have significantly more long-term variation than the other variables. Bihumeral width and BMI have the largest short-term variability and standing height has the smallest. Correlations between long-term variability and adult size and timing and intensity of the PS were small.
Conclusions:
A useful way of assessing long-term and short-term variability of SDS's, which is widely applicable has been described and applied to data relating to the growth of children. The results of this analysis are intriguing. Why is the underlying growth process of girls more variable than that of boys? Differences across skeletal parameters are also interesting and deserve further consideration.