> 
> Okay, I've tried the easy solutions to this, (namely VLOOKUP), but my brain
> doesn't do spreadsheets well.  I have two spreadsheets with names in column
> A.  The first spreadsheet is otherwise empty, but has month/year as column
> headers in row 1.  The second sheet has dates in column B.  I want to put
> "yes" in the first spreadsheet in cases where there is a corresponding date
> in the second sheet.  The catch: spreadsheet 2 has non-uniques.  That is:
> 
>      Spreadsheet 1                             Spreadsheet 2
> 
>    Name        Jan-97   Feb-97  Mar-97         Name       Date
> Bob                                          Carol        1/22/97
> Carol                                        Dave         1/24/97
> Dave                                         Dave         2/28/97
> Eddie                                        Frank        3/1/97
> Frank
> 
> My result should look like:
> 
>         Spreadsheet 1
> 
>     Name       Jan-97   Feb-97   Mar-97
> Bob
> Carol           Yes
> Dave            Yes      Yes
> Eddie
> Frank                             Yes
>

It can be done with array formula. Enter next text in cell b2:

{=summ(if((sheet2!$a$2:$a$5=$a2)*(sheet2!$b$2:$b$5>=b$1)*(sheet2!$b$2:$b$5