R: Convert a curved ERGM into a corresponding "fixed" ERGM.

fix.curved {ergm} | R Documentation

## Convert a curved ERGM into a corresponding "fixed" ERGM.

### Description

The generic `fix.curved` converts an `ergm` object or formula of a model with curved terms to the variant in which the curved parameters are fixed. Note that each term has to be treated as a special case.

### Usage

    fix.curved(object, ...)

    ## S3 method for class 'ergm'
    fix.curved(object, ...)

    ## S3 method for class 'formula'
    fix.curved(object, theta, ...)

### Arguments

`object` | An `ergm` object or an ERGM formula. The curved terms of the given formula (or the formula used in the fit) must have all of their arguments passed by name.
`...` | Unused at this time.
`theta` | Curved model parameter configuration.

### Details

Some ERGM terms such as `gwesp` and `gwdegree` have two forms: a curved form, for which their decay or similar parameters are to be estimated, and whose canonical statistics is a vector of the term's components (`esp(1)`, `esp(2)`, ... and `degree(1)`, `degree(2)`, ..., respectively) and a "fixed" form where the decay or similar parameters are fixed, and whose canonical statistic is just the term itself. It is often desirable to fit a model estimating the curved parameters but simulate the "fixed" statistic.

This function thus takes in a fit or a formula and performs this mapping, returning a "fixed" model and parameter specification. It only works for curved ERGM terms included with the `ergm` package. It does not work with curved terms not included in ergm.

### Value

A list with the following components:

`formula` | The "fixed" formula.
`theta` | The "fixed" parameter vector.

### See Also

`ergm`, `simulate.ergm`

### Examples

    data(sampson)
    gest<-ergm(samplike~edges+gwesp(),
               control=control.ergm(MCMLE.maxit=2))
    summary(gest)
    # A statistic for esp(1),...,esp(16)
    simulate(gest,output="stats")

    tmp<-fix.curved(gest)
    tmp
    # A gwesp() statistic only
    simulate(tmp$formula, coef=tmp$theta, output="stats")

[Package ergm version 4.2.2 Index]