collapse_W {eRm}  R Documentation 
Collapses columns of a design matrix for LLRA to specify different
parameter restrictions in LLRA
.
collapse_W(W, listItems, newNames)
W 
A design matrix (for LLRA), typically from a call to

listItems 
A list of numeric vectors. Each component of the list specifies columns to be collapsed together. 
newNames 
An (optional) character vector specifying the names of the collapsed effects. 
This function is a convenience function to collapse a design matrix,
i.e. to specify linear trend or treatment effects and so on. Collapsing
here means that effects in columns are summed up. For this, a list of numeric
vectors with the column indices of columns to be collapsed have to be
passed to the function. For example, if you want to collapse column 3, 6
and 8 into one new effect and 1, 4 and 9 into another it needs to be
passed with list(c(3,6,8),c(1,4,9))
.
The new effects can be given names by passing a character vector to the function with equal length as the list.
An LLRA design matrix as described by Hatzinger and Rusch
(2009). This can be passed as the W
argument to LLRA
or
LPCM
.
Thomas Rusch
Hatzinger, R. and Rusch, T. (2009) IRT models with relaxed assumptions in eRm: A manuallike instruction. Psychology Science Quarterly, 51, pp. 87–120.
The function to build design matrices from scratch, build_W
.
##An LLRA with 2 treatment groups and 1 baseline group, 5 items and 4
##time points. Item 1 is dichotomous, all others have 3, 4, 5, 6
##categories respectively.
llraDat2a < matrix(unlist(llraDat2[1:20]),ncol=4)
groupvec <rep(1:3*5,each=20)
W < build_W(llraDat2a, nitems=5, mpoints=4, grp_n=c(10,20,40), groupvec=groupvec,itmgrps=1:5)
#There are 55 parameters to be estimated
dim(W)
#Imposing a linear trend for the second item ,i.e. parameters in
#columns 32, 37 and 42 need to be collapsed into a single column.
collItems1 < list(c(32,37,42))
newNames1 < c("trend.I2")
Wstar1 < collapse_W(W,collItems1)
#53 parameters need to be estimated
dim(Wstar1)