mra (version 2.3)

ivar: Expand Individual-varying covariates in models

Description

Expands a vector of individual-varying values into a 2-d matrix of the appropriate size for use in MRA model formulas.

Usage

ivar(x, ns=attr(x,"ns"), drop.levels=attr(x,"drop.levels"))

Arguments

x

The vector of individual varying values to expand. This can be a factor (see as.factor). It is assumed that length(x) = number of individuals. If not, an error will occur in whatever routine called this functi

ns

Number of sampling occasions. Default is to use the 'ns' attribute of x. If ns is not specifed or is not an attribute of x, an error is thrown.

drop.levels

A vector of integers specifying which levels of a factor to drop. Only applicable if x is a factor. By default, the the 'drop.levels' attribute of x is used. If x does not have a 'drop.levels' at

Value

• A 2-d matrix of size length(x) x ns suitable for passing to the Fortran DLL of MRA for estimation. Values within rows are contant, values across rows vary according to x. If x is a factor, this matrix contains 0-1 indicator functions necessary to fit the factor. If x is a factor, attributes of the returned matrix are "levels" = levels of the factor and "contr" = contrasts used in the coding (always contr.treatment). For other contrast coding of factors, make your own 2-d matrix with a call to the appropriate function (like contr.poly).

See Also

F.cjs.estim, tvar

Examples

nan <- 30
ns <- 5
age <- as.factor(sample( c("J","S1","S2","Adult"), size=nan, replace=TRUE ))
attr(age,"ns") <- ns

# Note that levels get reordered (by R default, alphabetically)
attr(age,"drop.levels") <- (1:length(levels(age)))[ levels(age) == "J" ]

age.mat <- ivar(age)  # level J is the reference
age.mat <- ivar(age, drop=4) # level S2 is the reference

# Look at 3-D matrix produced when called with a factor.
dim(age.mat) <- c(nan,ns,length(levels(age))-1)
print(age.mat)  # each page is the 2-d matrix used in the fit.
print(age.mat[1,,])

age.mat <- ivar(age, drop=c(3,4))  # level S1 and S2 are combined and are the reference

# compare above to 
ivar( c(1,1,2,2,3,3), 5 )