End of analysisNext steps and reference material below. Not part of the analysis result.
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How do I find why CPA went up?
Split the change into volume, efficiency, and mix.
Method and FAQOpen only when you need the methodology
See the decomposition method and reading order
When to use it
Use it when weekly CPA rises without an obvious cause, when scaling one channel worsened overall efficiency, or when a report needs a defensible 'because of this'. You need a date column plus cost and outcome columns.
What it calculates
Decomposition runs once at the finest grain (channel × campaign × creative × day) and rolls up, so totals reconcile at every level. Mix weight uses outcome share, not cost share, and is centered on the overall average so the sign shows whether budget moved toward expensive units.
How to act
Read the top three movers first. Efficiency losses point to creative and targeting; mix shifts point to allocation. The decomposition is an accounting identity: it states how much each term contributed, not why it happened.
Frequently asked questions
Is the mix 'share' a cost share?
No, it is outcome share. A unit that improved efficiency can show a lower cost share and a higher outcome share, which looks wrong if read as spend.
Do totals differ between levels?
No. The decomposition happens once at the finest grain and rolls up by summation, so channel, campaign, and creative totals reconcile.
Does this prove the cause?
No. It attributes the change exactly to three terms; the underlying reason (bidding, competition, seasonality) still needs separate verification.