margins {ltm}R Documentation

Fit 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.
nprint a numeric value determining the number of margins with the largest residuals to be printed.
rule the rule of thumb used in determining the 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 can 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. The comparison is made using the so called chi-squared residuals. As a rule of thumb residuals greater than 3 (or 4) are indicative of poor fit. Analogous procedure we follow 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.

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
m <- ltm(Wirs~z1*z2)
margins(m, rule=3, nprint=2)
margins(m, "three", rule=3, nprint=2)


[Package ltm version 0.1-1 Index]