add1.vglm | R Documentation |

Compute all the single terms in the `scope`

argument that
can be
added to or
dropped from the model, fit those models and compute a
table of the changes in fit.

```
## S3 method for class 'vglm'
add1(object, scope, test = c("none", "LRT"), k = 2, ...)
## S3 method for class 'vglm'
drop1(object, scope, test = c("none", "LRT"), k = 2, ...)
```

`object` |
a fitted |

`scope, k` |
See |

`test` |
Same as |

`...` |
further arguments passed to or from other methods. |

These functions are a direct adaptation of
`add1.glm`

and
`drop1.glm`

for `vglm-class`

objects.
For `drop1`

methods, a missing `scope`

is taken to
be all terms in the model. 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’.

Compared to
`add1.glm`

and
`drop1.glm`

these functions are simpler, e.g., there is no
*Cp*, F and Rao (score) tests,
`x`

and `scale`

arguments.
Most models do not have a deviance, however twice the
log-likelihood differences are used to test the significance
of terms.

The default output table gives AIC, defined as minus twice log
likelihood plus `2p`

where `p`

is the rank of the model (the
number of effective parameters). This is only defined up to an
additive constant (like log-likelihoods).

An object of class `"anova"`

summarizing the differences
in fit between the models.

In general, the same warnings in
`add1.glm`

and
`drop1.glm`

apply here.
Furthermore, these functions have not been rigorously tested
for all models, so treat the results cautiously and please
report any bugs.

Care is needed to check that the constraint matrices of added
terms are correct.
Also, if `object`

is of the form
`vglm(..., constraints = list(x1 = cm1, x2 = cm2))`

then `add1.vglm`

may fail because the
`constraints`

argument needs to have the constaint
matrices for *all* terms.

Most VGAM family functions do not compute a deviance,
but instead the likelihood function is evaluated at the MLE.
Hence a column name `"Deviance"`

only appears for a
few models; and almost always there is a column labelled
`"logLik"`

.

`step4vglm`

,
`vglm`

,
`extractAIC.vglm`

,
`trim.constraints`

,
`anova.vglm`

,
`backPain2`

,
`update`

.

```
data("backPain2", package = "VGAM")
summary(backPain2)
fit1 <- vglm(pain ~ x2 + x3 + x4, propodds, data = backPain2)
coef(fit1)
add1(fit1, scope = ~ x2 * x3 * x4, test = "LRT")
drop1(fit1, test = "LRT")
fit2 <- vglm(pain ~ x2 * x3 * x4, propodds, data = backPain2)
drop1(fit2)
```

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