constrain.items: constrain item parameters in analysis of roll call data

Description

Sets constraints on specified item parameters in Bayesian analysis of roll call data by generating appropriate priors and start values for Markov chain Monte Carlo iterations.

Usage

constrain.items(obj, dropList = list(codes = "notInLegis", lop = 0),
                x, d = 1)

Arguments

obj

an object of class rollcall.

dropList

a list (or alist) indicating which voting decisions, legislators and/or roll calls are to be excluded from the subsequent analysis; see

a list containing elements with names matching votes found in dimnames(object$votes)[[2]] (but after any subsetting specified by dropList). Each component of the list mus

numeric, positive integer, the number of dimensions for which to set up the priors and start values.

Value

a list with elements:
xpprior means for ideal points. A matrix of dimensions number of legislators in obj by d.
xpvprior meansprecisions for ideal points. A matrix of dimensions number of legislators in obj by d.
bpprior means for item parameters. A matrix of dimensions number of items or votes in obj by d+1.
bpvprior meansprecisions for item parameters. A matrix of dimensions number of items or votes in obj by d+1.
xstartstart values for ideal points. A matrix of dimensions number of legislators in obj by d.
bstartstart values for ideal points. A matrix of dimensions number of items or votes in obj by d+1.

Details

constrain.items and its cousin, constrain.legis are usefully thougt of as pre-processor functions, generating priors and start values for both the item parameters and the ideal points. For the items specified in x, the prior mean for each dimension is set to the value given in x, and the prior precision for each dimension is set to 1e12 (i.e., a near-degenerate spike prior). For the other items, the priors are set to a mean of 0 and precision 0.01. All of the ideal points are given normal priors with mean 0, precision 1.

Start values are also generated for both ideal points and item parameters. The start values for the items specified in x are set to the values specified in x. The list resulting from constrain.items can then be given as the value for the parameters priors and startvals when ideal is run. The user is responsible for ensuring that a sufficient number of items are constrained such that when ideal is run, the model parameters are identified.

dropRollCall is first called to generate the desired roll call matrix. The entries of the roll call matrix are mapped to c(0,1,NA) using the codes component of the rollcall object. See the discussion in the documentation of ideal for details on the generation of start values.

Examples

Run this code

## run ideal with the default parameters
data(s109)
## run a 1d model, look for lack of fit
id1 <- ideal(s109,
             d=1,
             meanzero=TRUE,
             store.item=TRUE,
             maxiter=1e5,
             burnin=1e3,
             thin=1e2)  
id1sum <- summary(id1,include.beta=TRUE)

suspect1 <- id1sum$bSig[[1]]=="95close60 <- id1sum$bResults[[1]][,"Yea"] < 60
close40 <- id1sum$bResults[[1]][,"Yea"] > 40
suspect <- suspect1 & close60 & close40
id1sum$bResults[[1]][suspect,]
suspectVotes <- dimnames(id1sum$bResults[[1]][suspect,])[[1]]

## constraints on 2d model,
## close rollcall poorly fit by 1d model
## serves as reference item for 2nd dimension

cl <- constrain.items(s109,
                      x=list("2-150"=c(0,7),
                        "2-169"=c(7,0)),
                      d=2)

id1Constrained <- ideal(s109,
                        d=2,
                        meanzero=TRUE,
                        priors=cl,
                        startvals=cl,
                        maxiter=1e5,
                        burnin=1e3,
                        thin=1e2)
summary(id1Constrained,include.beta=TRUE)

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