I do this a lot in my reports. You can even tailor the layout of your headers and Detail areas using the same expression.

Enter this in the grouping expression box.

If you just have the two Choices:

=iif(parameters!Parametername.value='State', Fields!State.Value,Fields!Company.Value)

If you jave more than two

=iif(parameters!Parametername.value='State', Fields!State.Value,Fields!Company.Value)

iif(parameters!Parametername.value='Company', Fields!Company.Value,

Etc.

)

)

When I do something like this I also add a extra Header row and details row for each choince i the drop down.

Then I Set the Visibility I useing the same expression

=iif(parameters!Parametername.value='State', False,True) For the State Header and Details row

=iif(parameters!Parametername.value='State', False,True) For the Company Header and Detail Row

This gives you a lot of control over what gets displayed when either group is selected