[Numpy-discussion] Ticket #1223...
Thu Jul 1 12:43:50 CDT 2010
On Thu, Jul 1, 2010 at 9:11 AM, Charles R Harris
> On Thu, Jul 1, 2010 at 8:40 AM, Bruce Southey <email@example.com> wrote:
>> On 06/29/2010 11:38 PM, David Goldsmith wrote:
>> On Tue, Jun 29, 2010 at 8:16 PM, Bruce Southey <firstname.lastname@example.org>wrote:
>>> On Tue, Jun 29, 2010 at 6:03 PM, David Goldsmith
>>> <email@example.com> wrote:
>>> > On Tue, Jun 29, 2010 at 3:56 PM, <firstname.lastname@example.org> wrote:
>>> >> On Tue, Jun 29, 2010 at 6:37 PM, David Goldsmith
>>> >> <email@example.com> wrote:
>>> >> > ...concerns the behavior of numpy.random.multivariate_normal; if
>>> >> > of
>>> >> > interest to you, I urge you to take a look at the comments (esp.
>>> >> > :-) );
>>> >> > otherwise, please ignore the noise. Thanks!
>>> >> You should add the link to the ticket, so it's faster for everyone to
>>> >> check what you are talking about.
>>> >> Josef
>>> > Ooops! Yes I should; here it is:
>>> > http://projects.scipy.org/numpy/ticket/1223
>>> > Sorry, and thanks, Josef.
>>> > DG
>>> > _______________________________________________
>>> > NumPy-Discussion mailing list
>>> > NumPy-Discussion@scipy.org
>>> > http://mail.scipy.org/mailman/listinfo/numpy-discussion
>>> As I recall, there is no requirement for the variance/covariance of
>>> the normal distribution to be positive definite.
>> No, not positive definite, positive *semi*-definite: yes, the variance may
>> be zero (the cov may have zero-valued eigenvalues), but the claim (and I
>> actually am "neutral" about it, in that I wanted to reference the claim in
>> the docstring and was told that doing so was unnecessary, the implication
>> being that this is a "well-known" fact), is that, in essence (in 1-D) the
>> variance can't be negative, which seems clear enough. I don't see you
>> disputing that, and so I'm uncertain as to how you feel about the proposal
>> to "weakly" enforce symmetry and positive *semi*-definiteness. (Now, if you
>> dispute that even requiring positive *semi*-definiteness is desirable,
>> you'll have to debate that w/ some of the others, because I'm taking their
>> word for it that indefiniteness is "unphysical.")
>> >From http://en.wikipedia.org/wiki/Multivariate_normal_distribution
>> "The covariance matrix is allowed to be singular (in which case the
>> corresponding distribution has no density)."
>> So you must be able to draw random numbers from such a distribution.
>> Obviously what those numbers really mean is another matter (I presume
>> the dependent variables should be a linear function of the independent
>> variables) but the user *must* know since they entered it. Since the
>> function works the docstring Notes comment must be wrong.
>> Imposing any restriction means that this is no longer a multivariate
>> normal random number generator. If anything, you can only raise a
>> warning about possible non-positive definiteness but even that will
>> vary depending how it is measured and on the precision being used.
>> NumPy-Discussion mailing list
>> Mathematician: noun, someone who disavows certainty when their uncertainty
>> set is non-empty, even if that set has measure zero.
>> Hope: noun, that delusive spirit which escaped Pandora's jar and, with her
>> lies, prevents mankind from committing a general suicide. (As interpreted
>> by Robert Graves)
>> NumPy-Discussion mailing listNumPy-Discussion@scipy.orghttp://mail.scipy.org/mailman/listinfo/numpy-discussion
>> As you (and the theory) say, a variance should not be negative - yeah
>> right :-) In practice that is not exactly true because estimation procedures
>> like equating observed with expected sum of squares do lead to negative
>> estimates. However, that is really a failure of the model, data and
>> I think the issue is really how numpy should handle input when that input
>> is theoretically invalid.
> I think the svd version could be used if a check is added for the
> decomposition. That is, if cov = u*d*v, then dot(u,v) ~= identity. The
> Cholesky decomposition will be faster than the svd for large arrays, but
> that might not matter much for the common case.
Well, I'm not sure if what we have so far implies that consensus will
possibly be impossible to reach, so I'll just rest on my laurels (i.e., my
proposed compromise solution); just let me know if the docstring needs to be
changed (and how).
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