Calculated Field for Overall Average

Hello All,

I'm trying to make a calculated field that calculates the overall average to use as a reference line in a chart. This is what I have:

SUM([Visits w/ Tobacco Screening])/SUM([Visits w/ Vitals])*100

I don't want the value of this calculated field to change when the user selects different sites from the Site filter. As it is it changes when I select a different site. Anybody know what I'm doing wrong?

Thanks,

Brian