This document provides some additional guidelines to help you incorporate and optimize reach and frequency metrics.
Differences between assumed frequency for ROI and for optimization
There are differences between the assumed frequency for ROI and for optimization. You can adjust the assumed frequency for optimization if needed.
As discussed in ROI, mROI, and response curves, ROI measures the return of investment of a channel as it was executed during the time window for which the MMM has data. How a channel was executed includes how impressions are allocated across geos and time, and also includes the historical frequency of that channel.
Optimization assumes that future campaigns will be executed at the optimal frequency, since frequency is something often in an advertiser's control, especially for digital channels. If the optimal frequency is different from the historical frequency, a channel's performance in optimized budget allocation might not match the channel's historical performance according to ROI. This can be exacerbated if the current frequency is far from the optimal frequency.
If future campaigns won't be executed at the optimal frequency, you can use the optimization option to change the assumed frequency. This can be helpful for channels that cannot be executed at a specific average frequency.