Why blended CAC hides more than it reveals

Blended CAC, total spend across every channel divided by total new customers, is the number that ends up in the board deck because it's simple and comparable month over month. It's also a number that actively hides the information you'd need to make a good budget allocation decision.

A blended number can hold steady while masking real divergence underneath: paid search CAC improving while paid social CAC quietly deteriorates, with the two effects canceling out in the blended figure and giving no signal that a reallocation is overdue.

The fix isn't abandoning the blended number, it's refusing to make allocation decisions from it alone: channel-level CAC, tracked with the same rigor as the blended figure, is what actually tells you where the next dollar should go, and it's routinely a five-minute pull that most accounts simply don't do on a regular cadence.

A blended CAC that looks healthy can be sitting on top of one channel quietly subsidizing another's decline. The number is fine for a board slide. It's the wrong number to actually run the account on.