Why an average CPA is the wrong question

A campaign with a CPA of 40 euros can describe two very different accounts: one that sits stably around 40 euros everywhere, and one that hits 15 euros on desktop during the day and 90 on mobile in the evening. The average is the same number in both cases, and the wrong question to ask in both cases.

The question that does work is a breakdown: by time of day, by device, and where relevant by audience. Not because more detail is always better, but because a bid strategy at account level reacts to the average, while costs are generated by time of day and device. A tROAS target that fits the average therefore does not really fit either segment.

In practice this means: before adjusting a bid strategy, first split the period where the problem occurred by time of day and device. A spike confined to a few evening hours on mobile calls for a different fix than a structurally too-high CPA across the whole day. Ctrl PPC's 6-step Decision Framework builds on this: the hypothesis that follows from a signal points to the segment where the problem actually sits, not to the account as a whole.