How to fix parameters for some items with the `mirt` package in R? - Stack Overflow

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This is the follow-up question from this question

How can I fix parameters for some items but not all?

# Simulate data
set.seed(1)
dat <- matrix(sample(c(0,1), 100, TRUE), ncol = 5)
colnames(dat) <- paste0('item', 1:5)

# Fit a model and extract parameters of items
library(mirt)
mod <- mirt(dat, model = 1, itemtype = '2PL')
param <- mod2values(mod)
param$est <- F

# Works with the former data
mod2 <- mirt(dat, model = 1, itemtype = '2PL', pars = param)

# Doesn't work with new columns
new.data <- cbind(dat, item6 = sample(c(0,1), 20, TRUE))
mod3 <- mirt(new.data, model = 1, itemtype = '2PL', pars = param)

# Error: itemtype specification is not the correct length
# In addition: Warning messages:
# 1: EM cycles terminated after 500 iterations. 
# 2: In itemtype %in% c("2PLNRM", "3PLNRM", "3PLuNRM", "4PLNRM") & K <  :
#   longer object length is not a multiple of shorter object length

This is the follow-up question from this question

How can I fix parameters for some items but not all?

# Simulate data
set.seed(1)
dat <- matrix(sample(c(0,1), 100, TRUE), ncol = 5)
colnames(dat) <- paste0('item', 1:5)

# Fit a model and extract parameters of items
library(mirt)
mod <- mirt(dat, model = 1, itemtype = '2PL')
param <- mod2values(mod)
param$est <- F

# Works with the former data
mod2 <- mirt(dat, model = 1, itemtype = '2PL', pars = param)

# Doesn't work with new columns
new.data <- cbind(dat, item6 = sample(c(0,1), 20, TRUE))
mod3 <- mirt(new.data, model = 1, itemtype = '2PL', pars = param)

# Error: itemtype specification is not the correct length
# In addition: Warning messages:
# 1: EM cycles terminated after 500 iterations. 
# 2: In itemtype %in% c("2PLNRM", "3PLNRM", "3PLuNRM", "4PLNRM") & K <  :
#   longer object length is not a multiple of shorter object length

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1 Answer 1

Sorted by: Reset to default 1

The error occurs because param from the model with 5 items doesn’t include parameter specifications for the new sixth item. In other words, when you add a new item, mirt expects a parameter table that includes parameters for all items in the data.

So you can add this to your code:

param_new <- mod2values(mirt(new.data, model = 1, itemtype = '2PL'))

# Replace parameters for items from the old table
# (if you want to keep params for item1 to item5)
for(i in paste0("item", 1:5)){
  param_new[param_new$item == i, ] <- param[param$item == i, ]
}

mod3 <- mirt(new.data, model = 1, itemtype = '2PL', pars = param_new)

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