BICadj: Ways to compare SITAR models for fit

In statist7/sitar: Super Imposition by Translation and Rotation Growth Curve Analysis

Ways to compare SITAR models for fit

Description

BICadj and AICadj calculate the BIC and AIC for SITAR models, adjusting the likelihood for Box-Cox transformed y variables. varexp calculates the variance explained by SITAR models, compared to the corresponding fixed effect models. getL is used by [AB]ICadj to find what power the y variable is raised to.

Usage

BICadj(..., pattern = NULL)

AICadj(..., k = 2, pattern = NULL)

varexp(..., pattern = NULL)

getL(expr)

Arguments

...	one or more SITAR models.
pattern	regular expression defining names of models.
k	numeric, the penalty per parameter to be used; the default k = 2 is the classical AIC.
expr	quoted or unquoted expression containing a single variable name.

Details

The deviance is adjusted if the y variable is power-transformed, using the formula

adjusted deviance = deviance - 2n ( (\lambda-1) * log(gm) + % log(abs(\lambda)) )

where \lambda is the power transform, and n and gm are the length and geometric mean of y.

The variance explained is given by

\% explained = 100 * (1 -% (\sigma_2/\sigma_1)^2)

where \sigma_1 is the fixed effects RSD and \sigma_2 the SITAR random effects RSD.

BICadj and AICadj accept non-sitar models with a logLik class. varexp ignores objects not of class sitar.

getL does not detect if the variable in expr, or its log, contains a multiplying constant, so that the expressions log(x) and 1 + 2 * log(3 * x) both return 0.

Value

For BICadj and AICadj a named vector of deviances in increasing order. For varexp a named vector of percentages in decreasing order. For getL the power the variable in expr is raised to, or NA if expr is not a power of (a multiple of) the variable.

Author(s)

Tim Cole [e-mail]

See Also

BIC, AIC

Examples

data(heights)
## fit sitar model for height
m1 <- sitar(x=age, y=height, id=id, data=heights, df=5)

## update it for log(height)
m2 <- update(m1, y=sqrt(height))

## compare variance explained in the two models
varexp(m1, m2)

## compare BIC adjusting for sqrt transform
## the pattern matches names starting with "m" followed by a digit
BICadj(pattern="^m[0-9]")

## find what power height is raised to
getL(quote(sqrt(sqrt(height))))

statist7/sitar documentation built on Feb. 7, 2024, 2:08 a.m.