Yahoo! conducted usability studies in July 2008 to understand the Yahoo! user experience while navigating
the OpenID journey from the Relying Party to Yahoo! and back. The participants were all
experienced Yahoo! users who were tasked with signing into to a 3rd party site using their Yahoo! IDs
without having to create a new account for the site.

Download: Yahoo! OpenID: One Key, Many Doors (PDF - 1.4mb)

via Y! Developer