add1.BTm {BradleyTerry2} | R Documentation |

## Add or Drop Single Terms to/from a Bradley Terry Model

### Description

Add or drop single terms within the limit specified by the `scope`

argument. For models with no random effects, compute an analysis of deviance
table, otherwise compute the Wald statistic of the parameters that have been
added to or dropped from the model.

### Usage

```
## S3 method for class 'BTm'
add1(object, scope, scale = 0, test = c("none", "Chisq", "F"), x = NULL, ...)
```

### Arguments

`object` |
a fitted object of class inheriting from |

`scope` |
a formula specifying the model including all terms to be considered for adding or dropping. |

`scale` |
an estimate of the dispersion. Not implemented for models with random effects. |

`test` |
should a p-value be returned? The F test is only appropriate for models with no random effects for which the dispersion has been estimated. The Chisq test is a likelihood ratio test for models with no random effects, otherwise a Wald test. |

`x` |
a model matrix containing columns for all terms in the scope.
Useful if |

`...` |
further arguments passed to |

### Details

The hierarchy is respected when considering terms to be added or dropped: all main effects contained in a second-order interaction must remain, and so on.

In a scope formula ‘.’ means ‘what is already there’.

For `drop1`

, a missing `scope`

is taken to mean that all terms in
the model may be considered for dropping.

If `scope`

includes player covariates and there are players with
missing values over these covariates, then a separate ability will be
estimated for these players in *all* fitted models. Similarly if there
are missing values in any contest-level variables in `scope`

, the
corresponding contests will be omitted from all models.

If `formula`

includes random effects, the same random effects structure
will apply to all models.

### Value

An object of class `"anova"`

summarizing the differences in fit
between the models.

### Author(s)

Heather Turner

### See Also

### Examples

```
result <- rep(1, nrow(flatlizards$contests))
BTmodel1 <- BTm(result, winner, loser,
~ throat.PC1[..] + throat.PC3[..] + (1|..),
data = flatlizards,
tol = 1e-4, sigma = 2, trace = TRUE)
drop1(BTmodel1)
add1(BTmodel1, ~ . + head.length[..] + SVL[..], test = "Chisq")
BTmodel2 <- update(BTmodel1, formula = ~ . + head.length[..])
drop1(BTmodel2, test = "Chisq")
```

*BradleyTerry2*version 1.1-2 Index]