mirt (version 1.3)

mixedmirt: Mixed effects modeling for MIRT models

Description

mixedmirt fits MIRT models using FIML estimation to dichotomous and polytomous IRT models conditional on fixed and random effect of person and item level covariates. This can also be understood as 'explanatory IRT' if only fixed effects are modeled, or multilevel/mixed IRT if random and fixed effects are included. The method uses the MH-RM algorithm exclusively. Additionally, computation of the log-likelihood can be sped up by using parallel estimation via mirtCluster.

Usage

mixedmirt(data, covdata = NULL, model, fixed = ~1, random = NULL,
  itemtype = "Rasch", itemdesign = NULL, constrain = NULL, pars = NULL,
  return.design = FALSE, SE = TRUE, draws = 5000, TOL = NULL, ...)

Arguments

data

a matrix or data.frame that consists of numerically ordered data, with missing data coded as NA

covdata

a data.frame that consists of the nrow(data) by K 'person level' fixed and random predictors

model

an object returned from mirt.model() declaring how the factor model is to be estimated. See mirt.model for more details

fixed

a right sided R formula for specifying the fixed effect (aka 'explanatory') predictors from covdata and itemdesign. To estimate the intercepts for each item the keyword items is reserved and automatically add

random

a right sided formula or list of formulas containing crossed random effects of the form v1 + ... v_n | G, where G is the grouping variable and v_n are random numeric predictors within each group. If no interc

itemtype

same as itemtype in mirt, expect does not support the following item types: c('PC2PL', 'PC3PL', '2PLNRM', '3PLNRM', '3PLuNRM', '4PLNRM')

itemdesign

a data.frame object used to create a design matrix for the items, where each nrow(itemdesign) == nitems and the number of columns is equal to the number of fixed effect predictors (i.e., item intercepts). By default an

constrain

a list indicating parameter equality constrains. See mirt for more detail

pars

used for parameter starting values. See mirt for more detail

return.design

logical; return the design matrices before they have (potentially) been reassigned?

SE

logical; while standard errors are always computed for the MHRM algorithm this option forces the number of iterations to be no less than 400 so that the information matrix is computed more accurately. An alternative approach would be to drop the

draws

the number of Monte Carlo draws to estimate the log-likelihood for the MH-RM algorithm. Default is 5000

TOL

convergence threshold for MH-RM; default is .001

...

additional arguments to be passed to the MH-RM estimation engine. See mirt for more detail

Details

For dichotomous response models, mixedmirt follows the general form

$$P(x = 1|\theta, \psi) = g + \frac{(u - g)}{1 + exp(-1 * [\mathbf{\theta a} + \mathbf{X \beta} + \mathbf{Z \gamma}])}$$

where X is a design matrix with associated $\beta$ fixed effect coefficients, and Z is a design matrix with associated $\gamma$ random effects. For simplicity and easier interpretation, the unique item intercept values typically found in $\mathbf{X \beta}$ are extracted and reassigned within mirt's 'intercept' parameters (e.g., 'd'). To observe how the design matrices are structured prior to reassignment and estimation pass the argument return.design = TRUE.

Polytomous IRT models follow a similar format except the item intercepts are automatically estimated internally, rendering the items argument in the fixed formula redundant and therefore must be omitted from the specification. If there are a mixture of dichotomous and polytomous items the intercepts for the dichotomous models are also estimated for consistency.

To simulate maximum a posteriori estimates for the random effects use the randef function.

See Also

anova-method, coef-method, summary-method randef, calcLogLik, mirtCluster, mirt

Examples

#make some data
set.seed(1234)
N <- 750
a <- matrix(rlnorm(10,.3,1),10,1)
d <- matrix(rnorm(10), 10)
Theta <- matrix(sort(rnorm(N)))
pseudoIQ <- Theta * 5 + 100  + rnorm(N, 0 , 5)
pseudoIQ <- (pseudoIQ - mean(pseudoIQ))/10  #rescale variable for numerical stability
group <- factor(rep(c('G1','G2','G3'), each = N/3))
data <- simdata(a,d,N, itemtype = rep('dich',10), Theta=Theta)
covdata <- data.frame(group, pseudoIQ)
#use parallel computing
mirtCluster()

#specify IRT model
model <- mirt.model('Theta = 1-10')

#model with no person predictors
mod0 <- mirt(data, model, itemtype = 'Rasch')

#group as a fixed effect predictor (aka, uniform dif)
mod1 <- mixedmirt(data, covdata, model, fixed = ~ 0 + group + items)
anova(mod0, mod1)
summary(mod1)
coef(mod1)

#same model as above in lme4
wide <- data.frame(id=1:nrow(data),data,covdata)
long <- reshape2::melt(wide, id.vars = c('id', 'group', 'pseudoIQ'))
library(lme4)
lmod0 <- glmer(value ~ 0 + variable + (1|id), long, family = binomial)
lmod1 <- glmer(value ~ 0 + group + variable + (1|id), long, family = binomial)
anova(lmod0, lmod1)

#model using 2PL items instead of Rasch
mod1b <- mixedmirt(data, covdata, model, fixed = ~ 0 + group + items, itemtype = '2PL')
anova(mod1, mod1b) #better with 2PL models using all criteria (as expected, given simdata pars)

#continuous predictor with group
mod2 <- mixedmirt(data, covdata, model, fixed = ~ 0 + group + items + pseudoIQ)
summary(mod2)
anova(mod1b, mod2)

#view fixed design matrix with and without unique item level intercepts
withint <- mixedmirt(data, covdata, model, fixed = ~ 0 + items + group, return.design = TRUE)
withoutint <- mixedmirt(data, covdata, model, fixed = ~ 0 + group, return.design = TRUE)

#notice that in result above, the intercepts 'items1 to items 10' were reassigned to 'd'
head(withint$X)
tail(withint$X)
head(withoutint$X) #no intercepts design here to be reassigned into item intercepts
tail(withoutint$X)

###################################################
### random effects
#make the number of groups much larger
covdata$group <- factor(rep(paste0('G',1:50), each = N/50))

#random groups
rmod1 <- mixedmirt(data, covdata, 1, fixed = ~ 0 + items, random = ~ 1|group)
summary(rmod1)
coef(rmod1)

#random groups and random items
rmod2 <- mixedmirt(data, covdata, 1, random = list(~ 1|group, ~ 1|items))
summary(rmod2)
eff <- randef(rmod2) #estimate random effects

#random slopes with fixed intercepts (suppressed correlation)
rmod3 <- mixedmirt(data, covdata, 1, fixed = ~ 0 + items, random = ~ -1 + pseudoIQ|group)
summary(rmod3)
(eff <- randef(rmod3))

###################################################
##LLTM, and 2PL version of LLTM
data(SAT12)
data <- key2binary(SAT12,
                   key = c(1,4,5,2,3,1,2,1,3,1,2,4,2,1,5,3,4,4,1,4,3,3,4,1,3,5,1,3,1,5,4,5))
model <- mirt.model('Theta = 1-32')

itemdesign <- data.frame(itemorder = factor(c(rep('easier', 16), rep('harder', 16))))

#notice that the 'fixed = ~ ... + items' argument is omitted
LLTM <- mixedmirt(data, model = model, fixed = ~ 0 + itemorder, itemdesign = itemdesign,
   SE = TRUE) # SE argument ensures that the information matrix is computed accurately
summary(LLTM)
coef(LLTM)
wald(LLTM)
L <- c(-1, 1, 0)
wald(LLTM, L) #first half not different from second

#compare to items with estimated slopes (2PL)
twoPL <- mixedmirt(data, model = model, fixed = ~ 0 + itemorder, itemtype = '2PL',
                   itemdesign = itemdesign)
#twoPL not mixing too well (AR should be between .2 and .5), decrease MHcand
twoPL <- mixedmirt(data, model = model, fixed = ~ 0 + itemorder, itemtype = '2PL',
                  itemdesign = itemdesign, technical = list(MHcand = 0.8))
anova(twoPL, LLTM) #much better fit
summary(twoPL)
coef(twoPL)

wald(twoPL)
L <- matrix(0, 1, 34)
L[1, 1] <- 1
L[1, 2] <- -1
wald(twoPL, L) #n.s.

##LLTM with item error term
LLTMwithError <- mixedmirt(data, model = model, fixed = ~ 0 + itemorder, random = ~ 1|items,
    itemdesign = itemdesign)
summary(LLTMwithError)
#large item level variance after itemorder is regressed; not a great predictor of item difficulty
coef(LLTMwithError)

###################################################
### Polytomous example

#make an arbitrary group difference
covdat <- data.frame(group = rep(c('m', 'f'), nrow(Science)/2))

#partial credit model
mod <- mixedmirt(Science, covdat, model=1, fixed = ~ 0 + group)
coef(mod)

#gpcm to estimate slopes
mod2 <- mixedmirt(Science, covdat, model=1, fixed = ~ 0 + group,
                 itemtype = 'gpcm')
summary(mod2)
anova(mod, mod2)

#graded model
mod3 <- mixedmirt(Science, covdat, model=1, fixed = ~ 0 + group,
                 itemtype = 'graded')
coef(mod3)