Diagnostic plots for assessing the normality of residuals the
generalized least squares fit are obtained. The `form`

argument
gives considerable flexibility in the type of plot specification. A
conditioning expression (on the right side of a `|`

operator)
always implies that different panels are used for each level of the
conditioning factor, according to a Trellis display.

```
# S3 method for gls
qqnorm(y, form, abline, id, idLabels, grid, …)
```

y

an object inheriting from class `"gls"`

, representing
a generalized least squares fitted model.

form

an optional one-sided formula specifying the desired type of
plot. Any variable present in the original data frame used to obtain
`y`

can be referenced. In addition, `y`

itself
can be referenced in the formula using the symbol
`"."`

. Conditional expressions on the right of a `|`

operator can be used to define separate panels in a Trellis
display. The expression on the right hand side of `form`

and to
the left of a `|`

operator must evaluate to a residuals
vector. Default is `~ resid(., type = "p")`

,
corresponding to a normal plot of the standardized residuals.

abline

an optional numeric value, or numeric vector of length two. If given as a single value, a horizontal line will be added to the plot at that coordinate; else, if given as a vector, its values are used as the intercept and slope for a line added to the plot. If missing, no lines are added to the plot.

id

an optional numeric value, or one-sided formula. If given as
a value, it is used as a significance level for a two-sided outlier
test for the standardized residuals (random effects). Observations with
absolute standardized residuals (random effects) greater than the
\(1 - value/2\) quantile of the standard normal distribution are
identified in the plot using `idLabels`

. If given as a one-sided
formula, its right hand side must evaluate to a logical, integer, or
character vector which is used to identify observations in the
plot. If missing, no observations are identified.

idLabels

an optional vector, or one-sided formula. If given as a
vector, it is converted to character and used to label the
observations identified according to `id`

. If given as a
one-sided formula, its right hand side must evaluate to a vector
which is converted to character and used to label the identified
observations. Default is the innermost grouping factor.

grid

an optional logical value indicating whether a grid should
be added to plot. Default depends on the type of Trellis plot used:
if `xyplot`

defaults to `TRUE`

, else defaults to
`FALSE`

.

…

optional arguments passed to the Trellis plot function.

a diagnostic Trellis plot for assessing normality of residuals.

```
# NOT RUN {
fm1 <- gls(follicles ~ sin(2*pi*Time) + cos(2*pi*Time), Ovary,
correlation = corAR1(form = ~ 1 | Mare))
qqnorm(fm1, abline = c(0,1))
# }
```

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