I have enjoyed XLD's sticky posts (parts 1-3) on the SUMPRODUCT function. This function has allowed me to streamline and shorten many of my complex formulas employed in my workbooks. I am interested in learning the OR and NOT conditions seem to be working so differently than the AND conditions. For example, I have a workbook which includes the following possible conditions in Column B:

"Teacher"
"NA"
"Unknown"
"" [blank]
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976

Why does this formula:

=SUMPRODUCT(--(MyContacts!$B$1:$B$4000={1968,1969,1970,1971,1972,1973,1974,1975,"Unknown"}))
produce the correct count, while this formula:

=SUMPRODUCT(--(MyContacts!$B$1:$B$4000<>{"Teacher","","NA",1967,1976}))
results in a count that is grossly distorted? What is it about the <> that causes the second formula to respond differently than the = in the first formula? Do <> conditions have to be tested individually?