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SITAR--a useful instrument for growth curve analysis.

Cole TJ, Donaldson MD, Ben-Shlomo Y - Int J Epidemiol (2010)

Bottom Line: In CHS, growth tempo was associated with insulin-like growth factor-1 measured 50 years later (P = 0.01, N = 1009).For the girls with TS randomized to receive oxandrolone from 9 years, velocity was substantially increased compared with placebo (P = 10(-8)).The SITAR growth curve model is a useful epidemiological instrument for the analysis of height in puberty.

View Article: PubMed Central - PubMed

Affiliation: MRC Centre of Epidemiology for Child Health, UCL Institute of Child Health, London, UK. tim.cole@ich.ucl.ac.uk

ABSTRACT

Background: Growth curve analysis is a statistical issue in life course epidemiology. Height in puberty involves a growth spurt, the timing and intensity of which varies between individuals. Such data can be summarized with individual Preece-Baines (PB) curves, and their five parameters then related to earlier exposures or later outcomes. But it involves fitting many curves.

Methods: We present an alternative SuperImposition by Translation And Rotation (SITAR) model, a shape invariant model with a single fitted curve. Curves for individuals are matched to the mean curve by shifting their curve up-down (representing differences in mean size) and left-right (for differences in growth tempo), and the age scale is also shrunk or stretched to indicate how fast time passes in the individual (i.e. velocity). These three parameters per individual are estimated as random effects while fitting the curve. The outcome is a mean curve plus triplets of parameters per individual (size, tempo and velocity) that summarize the individual growth patterns. The data are heights for Christ's Hospital School (CHS) boys aged 9-19 years (N = 3245, n = 129,508), and girls with Turner syndrome (TS) aged 9-18 years from the UK Turner Study (N = 105, n = 1321).

Results: The SITAR model explained 99% of the variance in both datasets [residual standard deviation (RSD) 6-7 mm], matching the fit of individually-fitted PB curves. In CHS, growth tempo was associated with insulin-like growth factor-1 measured 50 years later (P = 0.01, N = 1009). For the girls with TS randomized to receive oxandrolone from 9 years, velocity was substantially increased compared with placebo (P = 10(-8)).

Conclusions: The SITAR growth curve model is a useful epidemiological instrument for the analysis of height in puberty.

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Related in: MedlinePlus

The effect on Figure 2 of adjusting in turn for (a) size α (shifting curves vertically), (b) tempo β (shifting curves horizontally), (c) velocity γ (shrinking–stretching age scales), and (d) extreme outliers
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Figure 3: The effect on Figure 2 of adjusting in turn for (a) size α (shifting curves vertically), (b) tempo β (shifting curves horizontally), (c) velocity γ (shrinking–stretching age scales), and (d) extreme outliers

Mentions: Figure 2 shows the raw data for the CHS cohort, a series of superimposed growth curves colour-coded to identify individuals. The RSD about the fitted spline curve is 6.6 cm (data not shown). Figure 3 shows the incremental effect of adjusting for each of the SITAR parameters in turn, where each subject’s growth curve is plotted using the individually adjusted height and age scales. In Figure 3a, a random height offset (i.e. the size parameter α) is adjusted for, so that individual curves are shifted up or down to minimize the RSD, which is reduced to 2.3 cm. In Figure 3b, a random age offset is added (the tempo parameter β), which shifts individual curves left–right and reduces the RSD to 1.2 cm. Finally, in Figure 3c the age scaling is added (the velocity parameter γ), which shrinks–stretches individual curves and reduces the RSD to 0.8 cm. The three graphs show increasingly obvious spiky outliers above and below the main body of data, corresponding to height recording errors at the time of measurement. To adjust for this, Figure 3d trims residuals exceeding 2.4 cm in absolute value (n = 952 or 0.74%) so as to remove the spikes but retain the main body of data. This reduces the RSD to 0.7 cm.Figure 2


SITAR--a useful instrument for growth curve analysis.

Cole TJ, Donaldson MD, Ben-Shlomo Y - Int J Epidemiol (2010)

The effect on Figure 2 of adjusting in turn for (a) size α (shifting curves vertically), (b) tempo β (shifting curves horizontally), (c) velocity γ (shrinking–stretching age scales), and (d) extreme outliers
© Copyright Policy - creative-commons
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC2992626&req=5

Figure 3: The effect on Figure 2 of adjusting in turn for (a) size α (shifting curves vertically), (b) tempo β (shifting curves horizontally), (c) velocity γ (shrinking–stretching age scales), and (d) extreme outliers
Mentions: Figure 2 shows the raw data for the CHS cohort, a series of superimposed growth curves colour-coded to identify individuals. The RSD about the fitted spline curve is 6.6 cm (data not shown). Figure 3 shows the incremental effect of adjusting for each of the SITAR parameters in turn, where each subject’s growth curve is plotted using the individually adjusted height and age scales. In Figure 3a, a random height offset (i.e. the size parameter α) is adjusted for, so that individual curves are shifted up or down to minimize the RSD, which is reduced to 2.3 cm. In Figure 3b, a random age offset is added (the tempo parameter β), which shifts individual curves left–right and reduces the RSD to 1.2 cm. Finally, in Figure 3c the age scaling is added (the velocity parameter γ), which shrinks–stretches individual curves and reduces the RSD to 0.8 cm. The three graphs show increasingly obvious spiky outliers above and below the main body of data, corresponding to height recording errors at the time of measurement. To adjust for this, Figure 3d trims residuals exceeding 2.4 cm in absolute value (n = 952 or 0.74%) so as to remove the spikes but retain the main body of data. This reduces the RSD to 0.7 cm.Figure 2

Bottom Line: In CHS, growth tempo was associated with insulin-like growth factor-1 measured 50 years later (P = 0.01, N = 1009).For the girls with TS randomized to receive oxandrolone from 9 years, velocity was substantially increased compared with placebo (P = 10(-8)).The SITAR growth curve model is a useful epidemiological instrument for the analysis of height in puberty.

View Article: PubMed Central - PubMed

Affiliation: MRC Centre of Epidemiology for Child Health, UCL Institute of Child Health, London, UK. tim.cole@ich.ucl.ac.uk

ABSTRACT

Background: Growth curve analysis is a statistical issue in life course epidemiology. Height in puberty involves a growth spurt, the timing and intensity of which varies between individuals. Such data can be summarized with individual Preece-Baines (PB) curves, and their five parameters then related to earlier exposures or later outcomes. But it involves fitting many curves.

Methods: We present an alternative SuperImposition by Translation And Rotation (SITAR) model, a shape invariant model with a single fitted curve. Curves for individuals are matched to the mean curve by shifting their curve up-down (representing differences in mean size) and left-right (for differences in growth tempo), and the age scale is also shrunk or stretched to indicate how fast time passes in the individual (i.e. velocity). These three parameters per individual are estimated as random effects while fitting the curve. The outcome is a mean curve plus triplets of parameters per individual (size, tempo and velocity) that summarize the individual growth patterns. The data are heights for Christ's Hospital School (CHS) boys aged 9-19 years (N = 3245, n = 129,508), and girls with Turner syndrome (TS) aged 9-18 years from the UK Turner Study (N = 105, n = 1321).

Results: The SITAR model explained 99% of the variance in both datasets [residual standard deviation (RSD) 6-7 mm], matching the fit of individually-fitted PB curves. In CHS, growth tempo was associated with insulin-like growth factor-1 measured 50 years later (P = 0.01, N = 1009). For the girls with TS randomized to receive oxandrolone from 9 years, velocity was substantially increased compared with placebo (P = 10(-8)).

Conclusions: The SITAR growth curve model is a useful epidemiological instrument for the analysis of height in puberty.

Show MeSH
Related in: MedlinePlus