xyadj {sitar}

Adjust x and y variables for SITAR random effects

Description

xyadj Adjusts x and y and optionally v values for subject-specific random effects from a SITAR model.

Usage

xyadj(object, x, y = 0, v = 0, id, abc = NULL, tomean = TRUE)

Arguments

object

a SITAR model.

x

a vector of x coordinates. If missing, x and y and id are obtained from object.

y

a vector of y coordinates (default 0).

v

a vector of velocity coordinates (default 0).

id

a factor denoting the subject levels corresponding to x and y and v.

abc

a data frame containing random effects for a, b, c and d (default ranef(object)[id, ]).

tomean

a logical defining the direction of adjustment. TRUE (default) indicates that individual curves are translated and rotated to match the mean curve, while FALSE indicates the reverse, the mean curve being translated and rotated to match individual curves.

Details

When tomean = TRUE the x and y and v values are adjusted to

(x - xoffset - b<fixed> - b<random>) * exp(c<random>) + xoffset + b<fixed>

y - a<random> - d<random> * x

(v - d<random>) / exp(c<random>)

When tomean = FALSE they are adjusted to

(x - xoffset - b<fixed>) / exp(c<random>) + xoffset + b<fixed> + b<random>

y + a<random> + d<random> * x

v * exp(c<random>) + d<random>

In each case missing values of the fixed or random effects are set to zero.

Value

The list of adjusted values:

x

numeric vector.

y

numeric vector the same length as x, or NULL.

v

numeric vector the same length as x, or NULL.

Author(s)

Tim Cole [e-mail]

Examples

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

## plot unadjusted data as growth curves
plot(m1, opt='u')

## overplot with adjusted data as points
with(heights, points(xyadj(m1), col='red', pch = 19))