Use of Partial SSN as Authenticator

[Gary Flynn <flynngn () JMU EDU>]

I've been asked to back up my assertion that the use of
a portion of the SSN ( e.g. last four digits ) as an
authenticator should be avoided as much as the use of
the entire SSN.

Can anyone point me to regulations, best practice studies,
or other material which may back up or refute this
assertion?

thanks,

--
Gary Flynn
Security Engineer
James Madison University
www.jmu.edu/computing/security