The incomplete beta function is misleadingly described in the documentation

Developers
1

The user guide (stan-reference-2.17.0.pdf) describes the inc_beta function as being the first integral in section F.2. Stan actually implements the inc_beta function as the second integral, i.e. as the regularised incomplete beta function.

This is the same design choice made in several other languages (for instance, scipy implements it this way), but the documentation should make this clear!

2

Thanks much for reporting. I’ll fix for the next release.

The place we track these is in whatever the “next manual” issue is on stan-dev/stan. Here’s a link to the issue comment for this fix.

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