There is not a ‘best’ testing metric for everyone.
There is a best testing metric based upon what you are trying to accomplish.
We’ve put together a quick reference chart to easily let you see what testing metric you should be using:
|What do you want to do?||The metric you should use|
|Increase conversions||Conversion per Impression (CPI)|
|Increase visitors||Click Through Rate (CTR)|
|Increase engaged visitors||Conversion per Impression (CPI)|
|Get the most revenue possible||Revenue per Impression (RPI)|
|Improve quality scores||Click Through Rate (CTR)|
There are two noticeable metrics missing from this list: ROAS and CPA.
Those two metrics should be filtering metrics and not winning metrics.
For instance, if you want the most conversions possible at a $35 target CPA; then you would use CPA as a filter and remove any ad that is above the $35 CPA target and then of the ads left, the highest CPI would be the winner.
These are the cases to use filtering metrics:
|What do you want to do?||Filtering metric||Winning metric|
|Highest revenue above a specific ROAS||ROAS||Revenue per Impression (RPI)|
|Most conversions under a target CPA||CPA||Conversion per Impression (CPI)|