None versus MISSING sentinel -- request for design feedback

[Chris Angelico]

On Fri, Jul 15, 2011 at 8:46 PM, Steven D'Aprano
<steve+comp.lang.python at pearwood.info> wrote:
> In general, you have missing values in statistics because somebody wouldn't
> answer a question, and the Ethics Committee frowns on researchers torturing
> their subjects to get information. They make you fill out forms.
>

Which, then, is in support of an explicit "User chose not to answer
this question" MISSING value.

ChrisA