A recent comScore ARS study found that 52% of shifts in brand sales are attributable to the quality of creative – making creative far and away the single most impactful driver of sales change. For context, media planning elements like GRPs, wearout and continuity/flighting account for 13% of sales change and price, promotion, distribution and other elements account for the remaining 35%.
I agree with the fact that ad quality is an important measure, but also would be interested in finding out how positioning on the web page helps.