Boost logo

Boost :

From: John Maddock (john_at_[hidden])
Date: 2007-01-30 08:35:13

This is the first part, of what will probably end up a multi-part review.

First the headline - I do think that the library should be included in
Boost - although I do have some reservations as noted below.

This part-review covers the documentation and a few simple tests I've
conducted. Later I'll try writing some accumulators so that I can comment
better on the design.

Documentation review


I'm fairly sure that the whole accumulator_set_wrapper business could be
completely avoided with a TR1 conforming reference_wrapper - which
unfortunately we don't have in Boost just yet - none the less this may be
worthwhile mentioning.

The "push all the data in this sequence" idiom seems like a very common use
case to me, is it not worthwhile supporting this directly via a couple of
member functions:

template <class It>
accumulator_set& accumulator_set::push_range(It begin, It end);

template <class Seq>
accumulator_set& accumulator_set::push_sequence(Seq const& s);

Typo in "This will calcualte the left"

Statistic accumulators:


This is going to confuse folks - I know it's got Paul and I into trouble
already! You're returning the "Kurtosis Excess" rather than the "Kurtosis
Proper". In the Math Toolkit stats functions we called these functions
"kurtosis" and "kurtosis_excess", and I'd favour similar naming here, or at
least a strong warning to the effect that there are multiple definitions in
the literature - otherwise unwary users will get into all kinds of trouble.

Differing implementations?

feature_of and as_feature get mentioned in the tutorial, along with a
sentence to say that there can be different implementations of the same
feature, but there is no explanation of how this works, and the links to the
reference pages for as_feature and feature_of are distinctly detail free :-(

This all leads on to the algorithms used for the mean and variance etc: I
assume that these are of the "naive" variety? Perfectly satisfactory for
simple cases, but there are well known pathological situations where these

I'm also missing accessors for: the standard deviation, "unbiased" variance,
and the unbiased (N-1) standard deviation. I realise I could write these
myself, but they would seem to be common enough cases to warrant inclusion
in the library. As it stands, I had to jump through hoops to get data I
could compare with "known good values".

BTW, I assume that there are no accumulators for Autocorrelation Coefficient

Which leads on to a quick comparison I ran against the "known good" data
here: The test
program is attached, and outputs the relative error in the statistics
calculated. Each test is progressively harder than the previous one, the
output is:

PI data:
Error in mean is: 0
Error in SD is: 3.09757e-016

Lottery data:
Error in mean is: 6.57202e-016
Error in SD is: 0

Accumulator 2 data:
Error in mean is: 9.25186e-015
Error in SD is: 2.71685e-012

Accumulator 3 data:
Error in mean is: 5.82076e-016
Error in SD is: 0.0717315

Accumulator 4 data:
Error in mean is: 9.87202e-015
Error in SD is: -1.#IND

As you can see the calculated standard deviation gets progressively worse,
in the final case, the computed variance is actually -2, which is clearly
non-sensical (and hence the NaN when using it to compute the standard
deviation) :-(

I haven't tried to debug these cases: stepping through the accumulator code
is not something I would recommend to anyone frankly (having tried it

No doubt other torture tests could be constructed for the mean -
exponentially distributed data where you see one or two large values,
followed by a large number of very tiny values springs to mind - this is the
sort of situation that the Kahan summation algorithm gets employed for.

I'm not sure what one should do about this. These certainly illustrate the
kinds of trap that the unwary can fall into. It's particularly a problem
for the "scientist in a rush" who may not be either a C++ or statistical
expert (whether (s)he thinks they are or not!) but needs a library to
quickly and reliably calculate some stats. Better documentation would
certainly help, but guidance on (or provision of - perhaps by default)
better algorithms might be welcome as well.

That's all for now,

Regards, John.

Boost list run by bdawes at, gregod at, cpdaniel at, john at