Calculate combined CPA without averaging campaign CPAs

The denominator defines the report

Use combined CPA = sum of eligible spend / sum of eligible results. Eligibility means the same outcome definition, reporting period and attribution basis. A submitted application and a completed purchase should not share the denominator. Neither should a website purchase count and a platform's broader result count simply because both columns are called conversions. Keep a separate subtotal for each business question; label the source and definition beside it.

Educational example, not a client case: campaign A spends 100 currency units and records 10 compatible leads, giving CPA 10. Campaign B spends 900 and records 30 leads, giving CPA 30. Averaging 10 and 30 produces 20, but total spend of 1,000 divided by 40 leads produces 25. B accounts for more results and therefore receives more weight. Weighting campaign CPAs by their lead counts also gives 25, provided every required CPA exists.

That shortcut breaks for a zero-result campaign: its individual CPA is undefined, but its spend still belongs in the portfolio numerator. If campaign C spends another 200 with zero leads, the correct total becomes 1,200 / 40 = 30. Dropping C because its CPA is blank hides a real cost. Unknown results are different from observed zero results: an incomplete import must be marked partial, not silently filled with zero.

What a weighted total protects

• Keep a reconciliation row containing eligible spend, eligible results and calculated CPA. A reviewer should reproduce the answer without trusting a dashboard widget.
• Separate portfolio efficiency from changes in portfolio composition. A shift toward a more expensive market can raise the total even when each market improves.
• Retain excluded rows with a reason. This shows whether the report measures the entire account or only the portion with compatible, available data.

Build and reconcile the calculation

1. Write the outcome definition first: for example, unique accepted leads under one agreed qualification rule. Choose a mature date range and identify the source used for each count.
2. Export or inspect the relevant stored rows. Include zero-result rows; isolate missing days and incompatible outcome types. Reconcile campaign spend with the account total before calculating a ratio.
3. If currencies differ, convert spend using an explicitly selected dated-rate method before summing. Do not add native-currency amounts or average already converted CPAs without checking their denominators.
4. Calculate the total and the contribution of each segment. Show both raw totals and segment CPAs so that a large campaign cannot disappear behind the aggregate.
5. Compare periods using the same inclusion rules. In AdAce Ads, use available stored data in the permitted client context; ask AI for a read-only reconciliation and a textual explanation, without propose_change or other tool writes.

When the combined figure is misleading

Sources and further reading

• AdAce Ads: results and cost per result
• AdAce Ads: multi-currency reporting