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Data validation must not exist in list

Excel formula: Data validation must not exist in list
Generic formula 
=COUNTIF(list,A1)=0
Explanation 

Note: Excel has a built-in data validation rules for dropdown lists. This page explains how to create a custom validation rule when you want to *prevent* a user from entering a value in a list.

To allow only values that do not exist in a list, you can use data validation with a custom formula based on the COUNTIF function. In the example shown, the data validation applied to B5:B9 is:

=COUNTIF(list,B5)=0

where "list" is the named range D5:D7.

How this formula works

Data validation rules are triggered when a user adds or changes a cell value.

In this case, the COUNTIF function is part of an expression that returns TRUE when a value does not exist in a defined list. The COUNTIF function simply counts occurrences of the value in the list. As long as the count is zero, the entry will pass validation. If the count is not zero (i.e. the user entered a value from the list) validation will fail.

Note:  Cell references in data validation formulas are relative to the upper left cell in the range selected when the validation rule is defined, in this case B5.

Author 
Dave Bruns

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