collapse_W {eRm}

Convenient Collapsing of LLRA Design Matrix

Description

Collapses columns of a design matrix for LLRA to specify different parameter restrictions in LLRA.

Usage

collapse_W(W, listItems, newNames)

Arguments

W

A design matrix (for LLRA), typically from a call to build_W or component $W from LLRA or LPCM

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.

Details

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.

Value

An LLRA design matrix as described by Hatzinger and Rusch (2009). This can be passed as the W argument to LLRA or LPCM.

Author(s)

Thomas Rusch

References

Hatzinger, R. and Rusch, T. (2009) IRT models with relaxed assumptions in eRm: A manual-like instruction. Psychology Science Quarterly, 51, pp. 87–120.

See Also

The function to build design matrices from scratch, build_W.

Examples

##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)