Which methodology should you use to calculate sponsorship ROI?

Should we renew this agreement for another four years? Which of our partnerships is generating the greatest return? Where should we allocate our remaining budget? Reliable and valid sponsorship ROI calculation is the golden goose of successful campaign management.

Many agencies offer measurement and sponsorship ROI calculations as part of their service. However, with such a vested interest in the outcome, can any agency truly mark its own homework effectively?