R: Extract Dummy Points Used to Fit a Point Process Model

dummy.ppm {spatstat.core} | R Documentation

## Extract Dummy Points Used to Fit a Point Process Model

### Description

Given a fitted point process model, this function extracts the ‘dummy points’ of the quadrature scheme used to fit the model.

### Usage

      dummy.ppm(object, drop=FALSE)

### Arguments

`object` | fitted point process model (an object of class `"ppm"`).
`drop` | Logical value determining whether to delete dummy points that were not used to fit the model.

### Details

An object of class `"ppm"` represents a point process model that has been fitted to data. It is typically produced by the model-fitting algorithm `ppm`.

The maximum pseudolikelihood algorithm in `ppm` approximates the pseudolikelihood integral by a sum over a finite set of quadrature points, which is constructed by augmenting the original data point pattern by a set of “dummy” points. The fitted model object returned by `ppm` contains complete information about this quadrature scheme. See `ppm` or `ppm.object` for further information.

This function `dummy.ppm` extracts the dummy points of the quadrature scheme. A typical use of this function would be to count the number of dummy points, to gauge the accuracy of the approximation to the exact pseudolikelihood.

It may happen that some dummy points are not actually used in fitting the model (typically because the value of a covariate is `NA` at these points). The argument `drop` specifies whether these unused dummy points shall be deleted (`drop=TRUE`) or retained (`drop=FALSE`) in the return value.

See `ppm.object` for a list of all operations that can be performed on objects of class `"ppm"`.

### Value

A point pattern (object of class `"ppp"`).

### Author(s)

Adrian Baddeley Adrian.Baddeley@curtin.edu.au

and Rolf Turner r.turner@auckland.ac.nz

### See Also

`ppm.object`, `ppp.object`, `ppm`

### Examples

     data(cells)
     fit <- ppm(cells, ~1, Strauss(r=0.1))
     X <- dummy.ppm(fit)
     npoints(X)
     # this is the number of dummy points in the quadrature scheme

[Package spatstat.core version 2.4-4 Index]