A new targeting option is available to advertisers that engage in Remarketing which allows advertisers to reach qualified new customers who are ‘similar’ to their website visitors and customers, even if they have never been to the website before. This new feature is called Similar Users and is a targeting option available on the Google Display Network (GDN) through Google AdWords.
The advertising platform uses the current audience remarketing lists within your account to identify other users who have similar browsing activity on the GDN and automatically finds a larger or smaller audience based on similar traits or your remarketing lists. The more diverse your remarketing audience lists are, the broader and deeper the opportunity will be.
To implement Similar Users, create a new campaign targeting the Display Network and in the new Display Network tab, select Interests and Remarketing. Next, click Change Display Targeting and select Remarketing Lists. Click the box next to “Show Similar Audiences” and from there you will be able to select various Similar Users lists based on your current Remarketing lists.
Google says that the cost per acquisition (CPA) for Similar Users tends to be range between the CPA of your Remarketing and contextual keyword campaigns.