BUG: Fixes precision issues with stats.tukeylambda distribution

[JacobHass8]

This fixes the bug documented in issue scipy/#21370. For $\lambda < 0$ and large $x$, the stats.tukeylambda.cdf function would not return 0. By reducing the parameter tukey_EPS, this can be achieved.

[mdhaber]

Thanks. Based on the original report in gh-21370, please add a test to this class:
https://github.com/scipy/scipy/blob/b1296b9b4393e251511fe8fdd3e58c22a1124899/scipy/stats/tests/test_distributions.py#L8371
and confirm that it fails before the fix and passes after.

Maybe name it test_gh21370_cdf_sf_saturation and as the first line, include a comment summarizing the issue and what the test checks. I don't foresee a need for parameterize; you can use an array for x (and lam if necessary), and tests of the CDF and SF can go on separate lines.

[JacobHass8]

Thanks. Based on the original report in gh-21370, please add a test to this class: https://github.com/scipy/scipy/blob/b1296b9b4393e251511fe8fdd3e58c22a1124899/scipy/stats/tests/test_distributions.py#L8371 and confirm that it fails before the fix and passes after.

Will do! Should I submit another PR in the scipy repository with this fix? I'm a little confused about how to go about this when xsf is a separate repository.

[mdhaber]

Oh, I didn't notice this was in xsf and not SciPy. @steppi can guide you on this one. After it's merged, I think you'll want to open a PR in SciPy that updates the xsf submodule commit and adds the test.

[JacobHass8]

Oh, I didn't notice this was in xsf and not SciPy. @steppi can guide you on this one. After it's merged, I think you'll want to open a PR in SciPy that updates the xsf submodule commit and adds the test.

I've added a PR in scipy that tests this case (scipy/scipy/#24167). Right now it will fail, but after this PR it will pass.

[steppi]

I think I need to take a closer look at this. I'm not confident at a glance that changing the value of tukey_EPS won't have other consequences. Also, @dschmitz89, is this something that SciPy could use Boost for instead?

[WarrenWeckesser]

I worked on tukeylambda last year, but it turned into a bit of a can of worms. I'll get back to it, and continue the discussion over in scipy/scipy#21370.

[JacobHass8]

I think I need to take a closer look at this. I'm not confident at a glance that changing the value of tukey_EPS won't have other consequences.

Certainly this would slow down convergence. I've seen it can take ~10 more iterations to converge to a solution with the new tukey_EPS value. I don't think it would never converge though. All the tests in scipy.stats still passed when I ran them.

[dschmitz89]

I think I need to take a closer look at this. I'm not confident at a glance that changing the value of tukey_EPS won't have other consequences. Also, @dschmitz89, is this something that SciPy could use Boost for instead?

I am afraid boost does not include the Tukey-Lambda distribution.

[JacobHass8]

I think I need to take a closer look at this. I'm not confident at a glance that changing the value of tukey_EPS won't have other consequences.

Would allowing tukey_EPS to be smaller in the tails be a good compromise as in

if (x > 1e9){
    detail::tukey_EPS = 1.0e-18;
}

There will certainly be a change in the cdf value about $x=10^9$, but surely it won't be that noticeable. I picked this value arbitrarily so it can certainly be changed.