Chip Pearson has lots of techniques to work with duplicates:
http://www.cpearson.com/excel/Duplicates.aspx

gh wrote:

I have 2 spreadsheets, with an id column that is assigned to a person,
in each one. One spreadsheet shows the people who pre-registered for an
event and the other spreadsheet shows the id of the people who attended.
I need to compare those who were registered against those who
attended, to show who was registered that did not show and those who
showed up without pre-registering. Can this be done in Excel?

TIA

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Dave Peterson