[Rd] Fix for bug in arima function

[Patrick Perry]

I noticed that the 3.2.1 release cycle is about to start.  Is there any
chance that this fix will make it into the next version of R?

This bug is fairly serious: getting the wrong variance estimate leads to
the wrong log-likelihood and the wrong AIC, BIC etc, which can and does
lead to suboptimal model selection.  If it's not fixed, this issue will
affect every student taking our time series course in Fall 2015 (and
probably lots of other students in other time series courses).  When I
taught time series in Spring 2015, I had to teach students how to work
around the bug, which wasted class time and shook student confidence in R.
It'd be great if we didn’t have to deal with this issue next semester.

Again, the fix is trivial:

--- a/src/library/stats/R/arima.R
+++ b/src/library/stats/R/arima.R
@@ -211,8 +211,10 @@ arima <- function(x, order = c(0L, 0L, 0L),
         if(fit$rank == 0L) {
             ## Degenerate model. Proceed anyway so as not to break old code
             fit <- lm(x ~ xreg - 1, na.action = na.omit)
+            n.used <- sum(!is.na(resid(fit))) - length(Delta)
+        } else {
+            n.used <- sum(!is.na(resid(fit)))
         }
-        n.used <- sum(!is.na(resid(fit))) - length(Delta)
         init0 <- c(init0, coef(fit))
         ses <- summary(fit)$coefficients[, 2L]
         parscale <- c(parscale, 10 * ses)


The patch that introduced the bug (
https://github.com/wch/r-source/commit/32f633885a903bc422537dc426644f743cc645e0
) was designed to change the initialization for the optimization routine.
The proposed fix leaves the deliberate part of the patch unchanged (it
preserves the value of "init0").

--

Patrick Perry
Assistant Professor
Stern School of Business
New York University





On Tue, Apr 21, 2015 at 7:34 AM, peter dalgaard <pdalgd at gmail.com> wrote:

> The bug repository is like an elephant: It doesn't forget, but the
> gestation period is long.
>
> In the present case, it is clear that something is not right, but someone
> needs to have sufficient recall and insight to check that your proposed fix
> is not unfixing a deliberate change. We should get to it eventually. (For
> some value of "we" not including "me"...)
>
> -pd
>
> On 20 Apr 2015, at 18:34 , Patrick Perry <pperry at stern.nyu.edu> wrote:
>
> > There is currently a bug in the arima function. Namely, for arima models
> with differencing or seasonal differencing, the innovation variance
> estimator uses the wrong denominator whenever xreg is non-null. This is the
> case, for example, when fitting an ARIMA(p,1,q) model with a drift term
> (common in financial applications). I reported the bug (and a fix) at
> https://bugs.r-project.org/bugzilla3/show_bug.cgi?id=16278 , but my
> report may have fallen through the cracks due to the timing around the
> 3.2.0 release.
> >
> > The bug was introduced in the patch displayed here:
> >
> >
> https://github.com/wch/r-source/commit/32f633885a903bc422537dc426644f743cc645e0
> >
> > The fix is very simple:
> >
> >
> https://github.com/patperry/r-source/commit/c1701c05ad91d5631eef196c2007ad9897b01f85
> >
> > I’ve posted a script that demonstrates the bug at
> >
> > https://gist.github.com/patperry/90a388b056e09cf6a51b
> >
> > Please let me know if there’s anything I can do to help get this fix
> incorporated.
> >
> >
> > --
> > Patrick Perry
> > Assistant Professor
> > Stern School of Business
> > New York University
> >
> > ______________________________________________
> > R-devel at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-devel
>
> --
> Peter Dalgaard, Professor,
> Center for Statistics, Copenhagen Business School
> Solbjerg Plads 3, 2000 Frederiksberg, Denmark
> Phone: (+45)38153501
> Office: A 4.23
> Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com
>
>
>
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>
>
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>
>
>

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