My guess is that some of the "numbers" are text and some are real numbers.

If you insert a column, you can verify it with:

=isnumber(a1)
and drag down
(if your range is in column A)

One quick way to convert all "text numbers" to "number numbers" is to:
copy an empty cell
select your range to convert
edit|paste special|check add



KB wrote:

A range contains 1 through 6, followed by 1 through 8, followed by 1 through 7.
"Advanced filter, unique records only" returns some duplicates. Why?

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Dave Peterson