Question

My code below for replication purposes. I’ve created a two parameter model using the ltm package. I’ve also used the factor.scores function to calculate a z1 value for each response, which is the second to last row in the “response_pattern” object below.

My question is, how can I extract the z1 value from response_pattern object so that it is a vector that I can use elsewhere? It should be a vector of length 12, one z1 for each response.

  item1 <- cbind(1,0,1,0,1,1,0,0,0,0,1,0)
  item2 <- cbind(0,0,0,1,0,1,0,1,0,0,1,0)
  item3 <- cbind(1,0,1,0,1,1,0,1,1,0,0,0)
  item4 <- cbind(0,0,1,1,0,1,0,0,0,1,1,1)

  Items <- matrix(c(item1, item2, item3, item4), ncol=4)

  library(ltm)
  fit <- ltm(Items ~ z1) # two parameter IRT model 

  response_pattern <- factor.scores(fit, resp.patterns = Items, method = "EB", return.MIvalues = TRUE) #Gets a z score for each response
  response_pattern  

Thanks!

Answer 1

?str is one of the most useful tools in your R toolkit.

> str(response_pattern)
List of 6
 $ score.dat:'data.frame':  12 obs. of  8 variables:
  ..$ Item 1: num [1:12] 1 0 1 0 1 1 0 0 0 0 ...
  ..$ Item 2: num [1:12] 0 0 0 1 0 1 0 1 0 0 ...
  ..$ Item 3: num [1:12] 1 0 1 0 1 1 0 1 1 0 ...
  ..$ Item 4: num [1:12] 0 0 1 1 0 1 0 0 0 1 ...
  ..$ Obs   : num [1:12] 2 2 1 1 2 1 2 1 1 2 ...
  ..$ Exp   : num [1:12] 1.051 1.948 1.225 0.668 1.051 ...
  ..$ z1    : num [1:12] 0.46 -0.6 0.519 -0.326 0.46 ...
  ..$ se.z1 : num [1:12] 0.558 0.797 0.66 0.713 0.558 ...
 $ method   : chr "EB"
 $ B        : num 5
 $ call     : language ltm(formula = Items ~ z1)
 $ resp.pats: logi TRUE
 $ coef     : num [1:4, 1:2] 0.125 2.3196 0.0241 0.0197 14.6276 ...
  ..- attr(*, "dimnames")=List of 2
  .. ..$ : chr [1:4] "Item 1" "Item 2" "Item 3" "Item 4"
  .. ..$ : chr [1:2] "Dffclt" "Dscrmn"
 - attr(*, "class")= chr "fscores"


> response_pattern$score.dat$z1