For 1990 file, MSA codes are effective with the 1990 data year and are based
on the 1990 Census.  All other (geo) codes are effective with the 1982 data year
and are based on results of the 1980 Census.  (pg. 48 of 1990 documentation)
(Should be true of 1990-1993).


1994/docs/geocode.txt apply 1994-1995