sitar-package | R Documentation |

SITAR is a method of growth curve analysis, based on nlme, that estimates a single mean growth curve as a regression B-spline, plus a set of up to four fixed and random effects (a, b, c and d) (d was added in version 1.2.0) defining how individual growth curves differ from the mean curve. SITAR stands for SuperImposition by Translation And Rotation.

The package also contains some utility functions for the LMS method, as used to construct growth reference centiles (see gamlss).

Package: | sitar |

Type: | Package |

Version: | 1.0 |

Date: | 2013-09-23 |

License: | GPL-2 |

Effect a (or alpha) measures size, and is a random intercept relative to the spline curve intercept. Effect b (or beta) measures tempo, the timing of the growth process, and reflects a shift on the x scale relative to the mean. Effect c (or gamma) is velocity, and indicates how the x scale is stretched or shrunk reflecting the rate at which 'time' passes for individuals. Effect d is a rotation in the plane. The aim is for individual curves, adjusted for abcd to lie on top of (i.e. be superimposed on) the mean curve.

The package creates an object of class `sitar`

, based on nlme,
representing the nonlinear mixed-effects model fit. Generic functions such
as `print`

, `plot`

and `summary`

have methods to show the
results of the fit, along with `resid`

, `coef`

, `fitted`

,
`fixed.effects`

and `random.effects`

to extract some of its
components. The functions `AICadj`

, `BICadj`

and `varexp`

compare respectively the AIC, BIC and variance explained of a series of
models, taking into account any transformations of the y variable. Functions
`plotclean`

, `velout`

, `codeplot`

and `zapvelout`

are
useful to clean the data file.

Tim Cole tim.cole@ucl.ac.uk

The idea of SITAR growth curve analysis arose from the paper by Beath (2007) and was first described in Cole et al (2010). The other references describe applications of SITAR to a variety of data forms.

Beath KJ. Infant growth modelling using a shape invariant model with random efffects. Stat Med 2007;26:2547-64.

Cole TJ, Cortina Borja M, Sandhu J, et al. Nonlinear growth generates age changes in the moments of the frequency distribution: the example of height in puberty. Biostatistics 2008;9:159-71.

Cole TJ, Donaldson MD, Ben-Shlomo Y. SITARâ€“a useful instrument for growth curve analysis. Int J Epidemiol 2010;39:1558-66.

Gault EJ, Perry RJ, Cole TJ, et al. Effect of oxandrolone and timing of pubertal induction on final height in Turner's syndrome: randomised, double blind, placebo controlled trial. BMJ 2011;\-342:d1980.

Johnson L, Llewellyn CH, van Jaarsveld CHM, et al. Genetic and environmental influences on infant growth: prospective analysis of the Gemini twin birth cohort. PLoS ONE 2011;6:e19918.

Prentice A, Dibba B, Sawo Y, et al. The effect of prepubertal calcium carbonate supplementation on the age of peak height velocity in Gambian adolescents. Am J Clin Nutr 2012;96:1042-50.

Dean MC, Cole TJ. Human life history evolution explains dissociation between the timing of tooth eruption and peak rates of root growth. PLoS ONE 2013;8:e54534.

Cole TJ, Statnikov Y, Santhakumaran S, et al. Birth weight and longitudinal growth in infants born below 32 weeks gestation: a UK population study. Arch Dis Child Fetal Neonatal Ed 2014;99:F34-F40.

Cole TJ, Rousham EK, Hawley NL, et al. Ethnic and sex differences in skeletal maturation among the Birth to Twenty cohort in South Africa. Arch Dis Child 2014;100:138-43.

Cole TJ, Pan H, Butler GE. A mixed effects model to estimate timing and intensity of pubertal growth from height and secondary sexual characteristics. Ann Hum Biol 2014;41:7683.

Johnson L, van Jaarsveld CHM, Llewellyn CH, et al. Associations between infant feeding and the size, tempo and velocity of infant weight gain: SITAR analysis of the Gemini twin birth cohort. Int J Obes 2014;38:980-7.

Pizzi C, Cole TJ, Richiardi L, et al. Prenatal influences on size, velocity and tempo of iInfant growth: findings from three contemporary cohorts. PLoS ONE 2014;9:e90291.

Ward KA, Cole TJ, Laskey MA, et al. The Effect of Prepubertal Calcium Carbonate Supplementation on Skeletal Development in Gambian Boys-A 12-Year Follow-Up Study. J C E M 2014;99:3169-76.

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