Thanks for all of your help, Glenn, Roy and Nick.
The solutions suggested by Roy and Glenn work well. Thanks for Nick's
correction of my description of the problem.
Best wishes,
Mandy
On 6/8/09, Nick Cox <[email protected]> wrote:
> Roy's solution is good. I just want to underline that the title is
> wrong. The problem is clearly to do with excluding observations, not
> variables.
>
> Nick
> [email protected]
>
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