margins {ltm}R Documentation

Fit of the model on the margins

Description

Checks the fit on the two- and three-way margins for ltm and rasch objects.

Usage

margins(object, type = c("two-way", "three-way"), nprint = 3, 
                rule = 3.5)

Arguments

object an object inheriting either from class ltm or class rasch.
type the type of margins to be used. See Details for more info.
nprint a numeric value determining the number of margins with the largest Chi-squared residuals to be printed.
rule the rule of thumb used in determining the indicative goodness-of-fit.

Details

Rather than looking at the whole set of response patterns, we can look at the two- and three-way margins. In the case of the former, we construct the 2 times 2 contingency tables obtained by taking the variables two at a time. Comparing the observed and expected two-way margins is analogous to comparing the observed and expected correlations when judging the fit of a factor analysis model. In the case of Bernoulli variates, the comparison is made using the so called Chi-squared residuals. As a rule of thumb residuals greater than 3.5 are indicative of poor fit. For a more strict rule of thumb use the rule argument. The analogous procedure is followed for the three-way margins.

Value

An object of class margins with components,

margins an array containing the values of chi-squared residuals.
type the type of margins that were calculated.
nprint the value of the nprint argument.
combs all possible two- or three-way combinations of the items.
rule the value of the rule argument.
call a copy of the matched call of object.

Author(s)

Dimitris Rizopoulos dimitris.rizopoulos@med.kuleuven.be

References

Bartholomew, D. (1998) Scaling unobservable constructs in social science. Applied Statistics, 47, 1–13.

Bartholomew, D. and Knott, M. (1999) Latent Variable Models and Factor Analysis, 2nd ed. London: Arnold.

Bartholomew, D., Steel, F., Moustaki, I. and Galbraith, J. (2002) The Analysis and Interpretation of Multivariate Data for Social Scientists. London: Chapman and Hall.

See Also

ltm, rasch

Examples


## Two- and Three-way residuals for the Rasch model
m <- rasch(Lsat)
margins(m)
margins(m, "three")

## Two- and Three-way residuals for the two-factor model
m <- ltm(Wirs ~ z1 + z2)
margins(m)
margins(m, "three")

## Two- and Three-way residuals for the interaction model,
## use the value 3 as a rule of thumb and report in each 
## case the 4 combinations of items with the largest residuals
m <- ltm(Wirs ~ z1 * z2)
margins(m, rule = 3, nprint = 4)
margins(m, "three", rule = 3, nprint = 4)


[Package ltm version 0.3-0 Index]