Jessi,

You've entered this Data Validation stuff only in the cells of column C,
even though the formula looks at A and B, so validation error stops will
occur only with entries in column C.

Why should the result be <= 1 (rather than 2)?

If the SUMPRODUCT has summed more than 1 (one is for the row we're in, any
others are the duplicates), then we want to produce a FALSE in our
validation formula, since it wants TRUE to not throw up the raspberry.

By the way, we could have used

=SUM(($A$2:$A$10=A2)*($B$2:$B$10=B2)*($C$2:$C$10< ""))<=1

just as well -- we're not really using the PRODUCT part of SUMPRODUCT. But
would have to enter it as an array formula (Ctrl-Shift-Enter), if used in a
cell. Oddly, it works in Data Validation. Don't write back and ask why, on
account of I don't know.

--
Earl Kiosterud
mvpearl omitthisword at verizon period net
-------------------------------------------

wrote in message
oups.com...
As a followup... I am trying to understand the logic behind the
SumProduct formula, and I don't quite get it. Is it somewhere along
the lines of:

For each row: the formula will compare the values in column A (the
Date) and if it finds a match it will assign a value of 1 (true) to the
Date array; otherwise 0 (false). Then it moves to column B (the
student name), and if it finds more than one occurrence of the
student's name in column B, then it again assigns a value of 1;
otherwise 0. The process is repeated in column C (category) for
whether the cell contains a value. Then it adds the values together.

Now this is where I get lost:

Why should the result be <= 1 (rather than 2)? Because it will be
possible to have more than one occurrence of the same date in column A
(a value of 1); and more than one occurrence of the student's name in
column B (another value of 1). Only in column C do I want to restrict
a second matching sequence. So, if I have matches in columns A and
B, that would be 1 + 1 = 2. What am I missing?

Many, many thanks for your help!

Jessi