[pymvpa] zscore / ZScoreMapper: How to transform both datasets of a split equally

[Yaroslav Halchenko]

>     # Here the two mapped datasets should be zscored equally, but with

by 'equally' do you mean that mean/std should be computed on
concatenation of two datasets?

if it would be ok with you if mean/std gets computed based on training
portion of the dataset, then why not simply construct
MappedClassifier chaining your two mappers together? e.g. something like
(not tested)

clfr = MappedClassifier(SVM(),
                        mapper=ChainMapper([MyCustomMapper(...),
                                            ZScoreMapper(...)]))

and then simply doing 

cvte = CrossValidatedTransferError(TransferError(clfr),
                            NFoldSplitter(nperlabel="equal"),
                            enable_states = ['confusion'])

error = cvte(dataset)
print cvte.states.confusion

?

On Fri, 25 Feb 2011, Thorsten Kranz wrote:

> Hi all,

> I think this should be trivial, but maybe I'm not seeing the forest
> for the trees.

> I want to do a cross-validation, I do this manually, also involving a
> custom mapper, by (simplified Example):

> cm = ConfusionMatrix()
> clfr = SVM()
> for d1, d2 in NFoldSplitter(nperlabel="equal")(dataset):
>     mcm = MyCustomMapper(...)
>     mcm.train(d1)
>     d1_mapped = mcm.forward(d1)
>     d2_mapped = mcm.forward(d2)
>     # Here the two mapped datasets should be zscored equally, but with
> perchunk=False, pervoxel=True
>     ???
>     clfr.train(d1_mapped)
>     cm.add(clfr.predict(d2_mapped.samples),d2_mapped.labels)

> I don't see how to zscore both mapped datasets equally AND
> pervoxel=True, perchunk=False. I would love to use the ZScoreMapper
> for that, but it doesn'T provide the "per*" arguments. And the method
> zscore does accept parameters mean and std, but doesn't return them
> when called so I could reuse them in a second call.

> I guess I will write my own zscore-method, just copy-pasting it from
> your source, but returning mean and std.

> Or do you have another / better proposal?

> Greetings,
> Thorsten

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