tvar function

Expand Time-varying covariates in models

Expands a vector of time-varying values into a 2-d matrix for use in MRA model formulas.

tvar(x, nan=attr(x,"nan"), drop.levels=attr(x,"drop.levels"))

Arguments

• x: The vector of time varying values to expand. This can be a factor (see as.factor). It is assumed that length(x) = number of sampling occasions. If not, an error will occur in whatever routine called this function (e.g., F.3d.model.matrix).

• nan: Number of individuals. Default is to use the 'nan' attribute of x. If nan is not specified 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' attribute, the first level of the factor is dropped. drop.levels=length(levels(x))

  does the SAS thing and drops the last level of a factor. Specifying multiple levels to drop is acceptable. E.g., drop.levels=c(1,2,7) drops the 1st, 2nd, and 7th levels of the factor. First level of a factor is first element of levels(x). Second level of a factor is second element of levels(x), and so on. Setting drop.levels to 0, a negative number, or a number greater than the number of levels will not drop any levels (this is so-called cell mean coding). Keep in mind presence of the intercept.

Returns

A 2-d matrix of size nan x length(x) suitable for passing to the Fortran DLL of MRA for estimation. Values within columns are constant, values across columns 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).

Author(s)

Trent McDonald, WEST-INC, tmcdonald@west-inc.com

See Also

F.cjs.estim, tvar

Examples

nan <- 30
ns <- 5
time.occ <- as.factor(paste("T",1:ns, sep=""))
attr(time.occ,"nan") <- nan
attr(time.occ,"drop.levels") <- ns

time.mat <- tvar(time.occ) # Last occasion is the reference, the SAS and MARK default.

time.mat <- tvar(as.factor(1:ns),nan,ns) #equivalent to above.

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

#  compare above to 
tvar( 1:ns, nan )