Dear Colleagues,
I recently answered [this question]() on StackOverflow that identified
what seems to be unusual behaviour with `stats:::nls.fitted()`. In
particular, a null model returns a single fitted value rather than a
vector of the same fitted value of `length(y)`. The documentation
doesn’t make it seem like this is the intended behaviour, so I’m not
sure if it’s a bug, a “Wishlist” item or something that is working
as intended even though it seems unusual to me. I looked through the
bug reporting page on the R project website and it suggested contacting
the R-devel list in cases where the behaviour is not obviously a bug to
see whether others find the behaviour equally unusual and I should
submit a Wishlist item through Bugzilla.
Below is a reprex that shows how the fitted values of a model with just
a single parameter is length 1, but if I multiply that constant by a
vector of ones, then the fitted values are of `length(y)`. Is this
something that should be reported?
``` r
dat <-
data.frame(y=c(80,251,304,482,401,141,242,221,304,243,544,669,638),
ones = rep(1, 13))
mNull1 <- nls(y ~ a, data=dat, start=c(a=mean(dat$y)))
fitted(mNull1)
#> [1] 347.6923
#> attr(,"label")
#> [1] "Fitted values"
mNull2 <- nls(y ~ a*ones, data=dat, start=c(a=mean(dat$y)))
fitted(mNull2)
#> [1] 347.6923 347.6923 347.6923 347.6923 347.6923 347.6923 347.6923
347.6923
#> [9] 347.6923 347.6923 347.6923 347.6923 347.6923
#> attr(,"label")
#> [1] "Fitted values"
```
Created on 2023-01-25 by the [reprex
package](https://reprex.tidyverse.org) (v2.0.1)
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