pls -- crossval vs plsr(..., CV=TRUE)

martin peters writes:

$ library(pls)
$ data(NIR)

$ testing.plsNOCV <- plsr(y ~ X, 6, data = NIR, method="kernelpls", 
validation="none")
$ NIR.plsCV <- plsr(y ~ X, 6, data = NIR, CV=TRUE, method="kernelpls")
$ testing.plsCV <- crossval(testing.plsNOCV)
$ R2(NIR.plsCV)
(Intercept)      1 comps      2 comps      3 comps      4 comps      5 
comps
      0.0000       0.9812       0.9825       0.9964       0.9997       
0.9999
     6 comps
      0.9999
$ R2(testing.plsCV)
(Intercept)      1 comps      2 comps      3 comps      4 comps      5 
comps
      0.0000       0.9678       0.9782       0.9941       0.9991       
0.9996
     6 comps
      0.9997

[...]

If the above result is correct can someone explain the difference to me.

There are two reasons:

1) The call plsr(y ~ X, 6, data = NIR, CV=TRUE, method="kernelpls") is
   incorrect.  The `CV' argument of the superseded `pls.pcr' package
   has been replaced by the `validation' argument, so the correct call
   would be
   NIR.plsCV <- plsr(y ~ X, 6, data = NIR, validation="CV", method="kernelpls")
   (If you had done R2(testing.plsNOCV), you would have gotten exactly
   the same as with the R2(NIR.plsCV) above.)

2) plsr(... , validation = "CV") and crossval(...) both by default use
   CV with 10-fold _randomly selected_ segments, which means that each
   time you run the cross-validation, you will get slightly different
   results.  (Try running R2(crossval(testing.plsNOCV)) a couple of times.)

   If you want the same segments in two separate calls, either add the
   argument segment.type = "consecutive" or "interleaved", or specify
   the segments explicitly with the `segments' argument (see
   ?crossval or ?mvrCv for how).

   The segments actually used in a cross-validation is stored in the
   $validation$segments component of the object,
   i.e. testing.plsCV$validation$segments.

(By the way, `method = "kernelpls"' is not needed, as it is the
default fit method for plsr (and mvr).)

Bj??rn-Helge Mevik