Measurement

Myth:Platform-reported ROAS is sufficient evidence.

Evidence:Geo-based holdout experiments give unbiased incrementality estimates without user-level tracking and are the most practical causal method for most advertisers.

Myth:Your ad spend only builds demand for you.

Evidence:Randomized field experiments on a restaurant-search platform show ads also lift sales for non-advertised competitors; spillovers concentrate on same-cuisine, highly rated rivals and are largest when ad intensity is low.

Myth:Bidding on your own brand name is always wasted money.

Evidence:Bing field experiments across thousands of brands show brand ads add a small but real 1-4% click lift when no competitor bids, and become defensive when rivals bid to siphon the traffic.

Myth:A big enough A/B test or attribution platform can pin down each campaign's ROI.

Evidence:Across 25 large field experiments with major U.S. retailers and brokerages, the median confidence interval on ad ROI was over 100 percentage points wide; individual-level sales are so volatile (coefficient of variation ~10) that an informative test often needs 10M+ person-weeks.

Myth:Last-click attribution is a fair scorecard.

Evidence:Large-scale field experiments show branded search and retargeting capture credit for conversions that would have happened anyway; last-click can overstate paid value by an order of magnitude.