CBO hands 80% of budget to one ad set in 48h — usually the wrong one

CBO hands 80% of budget to one ad set in 48h — usually the wrong one

CBO (Campaign Budget Optimization) distributes spend across ad sets automatically. Left alone, it concentrates fast on early winners — before they're statistically real.

Budget concentration (35 CBO campaigns, 48h in):
— Top ad set share: 78% median
— Decision made on: 11 conversions median
— Sets killed before 50 conversions: 61%

It's deciding on noise, then starving sets that needed 3 more days.

Fix: use minimum spend limits per ad set (ad-set-level cost controls) so each gets enough data, or run separate ABO (ad-set-budget) campaigns for the test phase, then move winners to CBO.

Read: CBO is a great manager and a terrible judge — don't let it rule on 11 data points.