mixed-methods: Mixed-Format Response Probabilities
In plink: IRT Separate Calibration Linking Methods

Description Usage Arguments Details Value Author(s) See Also Examples

This function computes the probability of responding in a specific category for one or more items for a given set of theta values when the items are from a mixed-format test.

mixed(x, cat, poly.mod, theta, dimensions = 1, items, information = FALSE, angle, ...)

## S4 method for signature 'numeric', 'numeric'
mixed(x, cat, poly.mod, theta, dimensions, items, information, angle, ...)

## S4 method for signature 'matrix', 'numeric'
mixed(x, cat, poly.mod, theta, dimensions, items, information, angle, ...)

## S4 method for signature 'data.frame', 'numeric'
mixed(x, cat, poly.mod, theta, dimensions, items, information, angle, ...)

## S4 method for signature 'list', 'numeric'
mixed(x, cat, poly.mod, theta, dimensions, items, information, angle, ...)

## S4 method for signature 'irt.pars', 'ANY'
mixed(x, cat, poly.mod, theta, dimensions, items, information, angle, ...)

## S4 method for signature 'sep.pars', 'ANY'
mixed(x, cat, poly.mod, theta, dimensions, items, information, angle, ...)

`x`	an `R` object containing item parameters
`cat`	vector identifying the number of response categories for each item. If multiple-choice model items are included, `cat` for these items should equal the number of response categories plus one (the additional category is for 'do not know')
`poly.mod`	object of class `poly.mod` identifying the items associated with each IRT model
`theta`	vector, matrix, or list of theta values for which probabilities will be computed. If `theta` is not specified, an equal interval range of values from -4 to 4 is used with an increment of 0.5. See details below for more information.
`dimensions`	number of modeled dimensions
`items`	numeric vector identifying the items for which probabilities should be computed
`information`	logical value. If `TRUE` compute item information. In the multidimensional case, information will be computed in the directions specified by `angle` or default angles of 0 - 90 in increments of 10 degrees.
`angle`	vector or matrix of angles between the dimension 1 axis and the corresponding axes for each of the other dimensions for which information will be computed. When there are more than two dimensions and `angle` is a vector, the same set of angles will be used relative to each of the corresponding axes.
`...`	further arguments passed to or from other methods. See details below.

The item parameters supplied to this method can be associated with a single IRT model or multiple models. When the parameters are tied to only one model, the format of x (for either unidimensional or multidimensional models) should follow the conventions in drm for dichotomous response models (i.e. 1PL, 2PL, 3PL), gpcm for the partial credit model and generalized partial credit model, grm for the graded response model, mcm for the multiple-choice model, and nrm for the nominal response model. When the parameters are associated with two or more models, the parameters should be combined. See as.irt.pars or for more details on how the parameters from different models can be combined. Additional arguments for the above models can be passed to this method as well.

theta can be specified as a vector, matrix, or list. For the unidimensional case, theta should be a vector. If a matrix or list of values is supplied, they will be converted to a single vector of theta values. For the multidimensional case, if a vector of values is supplied it will be assumed that this same set of values should be used for each dimension. Probabilities will be computed for each combination of theta values. Similarly, if a list is supplied, probabilities will be computed for each combination of theta values. In instances where probabilities are desired for specific combinations of theta values, a j x m matrix should be specified for j ability points and m dimensions where the columns are ordered from dimension 1 to m.

The mixed function essentially compiles response probabilities computed using the functions drm, grm, gpcm, nrm, and mcm for the associated models respectively. All of the arguments specified in any one of these functions can be passed to mixed as an additional argument. For example, the argument incorrect can be passed to drm and catprob can be passed to grm. In the functions drm, grm, and gpcm there is an argument D for the value of a scaling constant. In mixed, a single argument D can be passed that will be applied to all applicable models, or arguments D.drm, D.grm, and D.gpcm can be specified for each model respectively. If an argument is specified for D and, say D.drm, the values for D.grm and D.gpcm (if applicable) will be set equal to D. If only D.drm is specified, the values for D.grm and D.gpcm (if applicable) will be set to 1.

Returns an object of class irt.prob

Jonathan P. Weeks weeksjp@gmail.com

plot: plot item characteristic/category curves
irt.prob, irt.pars, sep.pars: classes

###### Unidimensional Examples ######
# Compute probabilities for three dichotomous (3PL) items and two 
# polytomous (gpcm) items without a location parameter
dichot <- matrix(c(1.2, .8, .9, 2.3, -1.1, -.2, .24, .19, .13),3,3)
poly <- matrix(c(.64, -1.8, -.73, .45, NA, .88, .06, 1.4, 1.9, 2.6),
  2,5,byrow=TRUE)
pars <- rbind(cbind(dichot,matrix(NA,3,2)),poly)
cat <- c(2,2,2,4,5)
pm <- as.poly.mod(5, c("drm","gpcm"), list(1:3,4:5))
x <- mixed(pars, cat, pm) 
plot(x)

# Specify a different scaling constant for the GPCM items in the
# above example
x <- mixed(pars, cat, pm, D.gpcm=1.7) 
plot(x)


# Compute probabilities for three dichotomous (3PL) items, four 
# polytomous items, two gpcm items and two nrm items. Include a 
# location parameter for the gpcm items
a <- matrix(c(
  1.2, rep(NA,4),
  .8, rep(NA,4),
  .9, rep(NA,4),
  .64, rep(NA,4),
  .88, rep(NA,4),
  .905, .522, -.469, -.959, NA, 
  .828, .375, -.357, -.079, -.817),7,5,byrow=TRUE)
b <- matrix(c(
  2.3, rep(NA,4),
  -1.1, rep(NA,4),
  -.2, rep(NA,4),
  -.69, -1.11, -.04, 1.14, NA,
  1.49, -1.43, -.09, .41, 1.11,
  .126, -.206, -.257, .336, NA, 
  .565, .865, -1.186, -1.199, .993),7,5,byrow=TRUE)
c <- c(.14, .19, .26, rep(NA,4))
pars <- list(a,b,c)
cat <- c(2,2,2,4,5,4,5)
pm <- as.poly.mod(7, c("drm","gpcm","nrm"), list(1:3,4:5,6:7))
x <- mixed(pars, cat, pm, location=TRUE)
plot(x)

###### Multidimensional Example ######
# Compute response probabilities for four dichotomous items
# modeled using the M2PL and three polytomous items modeled
# using the multidimensional graded response model. For the 
# later items, cumulative probabilities are computed.
a <- matrix(c(1.66,1.72,.69,.19,.88,1.12,.68,1.21,
     .873, .226, .516, .380, .613, .286 ),7,2,byrow=TRUE)
d <- matrix(c(-.38,NA,NA,NA,NA,
     -.68,NA,NA,NA,NA,
     -.91,NA,NA,NA,NA,
     -1.08,NA,NA,NA,NA,
     2.255, 1.334, -.503, -2.051, -3.082,
     1.917, 1.074, -.497, -1.521, -2.589,
     1.624, .994, -.656, -1.978, NA),7,5,byrow=TRUE)
cat <- c(2,2,2,2,6,6,5)
pars <- cbind(a,d)
pm <- as.poly.mod(7,c("drm","grm"),list(1:4,5:7))
x <- mixed(pars, cat, pm, dimensions=2, catprob=TRUE)
plot(x)