Re: Test Failure

[Jason Stover]

On Mon, Apr 17, 2006 at 10:21:12AM -0700, Ben Pfaff wrote:
> The catch is that this isn't what chisq is actually computing.
> It is actually printing pdf.chisq(idf.chisq(0.01, 2), 2).
> Here is code that illustrates that[+]:
> 
...

Pardon me if this is too far off topic, but the fact that pspp
evaluates pdf.chisq(idf.chisq(0.01, 2), 2), and the fact that it
delays writing data to the active file has me wondering: Does pspp try
to delay any kind of evaluation until the 'end'? I have heard that
gurus view delayed evaluation (and the consequent fewer assignments)
favorably. Is that part of its design? (If my question makes no sense,
do not hesitate to say so.)

> So the real question may be, how should we adjust the chi-square
> test to avoid this edge case?  Perhaps we should just pick a
> different initial value of P that does not come so close to a
> rounding error.

I kind of like having the 'edge' cases in the test, partly
to show where the computation depends on architecture. Would
it be possible to leave those cases in the tests, but without causing
a test failure? Perhaps if the user wants to see the differences,
they could type 'make paranoid-check' or something.

> [+] Why is it done that way?  Because then I don't have to know
> the proper range of values of x for the various pdf, cdf,
> etc. functions.  I know the range of P and feeding it through idf
> gives me the proper range for x.  This is lazy, I admit.

I have no problem with the laziness, but maybe the test should
not fail unless the user wants to be persnickety.

-Jason