It’s been well known that Google has been working on a solution to tie clicks on search ads back to in-store traffic and sales as part of Estimated Total Conversions in AdWords. Today, the company announced the initial roll out of a new “Store Visits” metric in Estimated Total Conversions (ETC).

While e-commerce continues to grow, offline sales still account for more than 93 percent of total retail sales in the US. As consumers increasingly turn to their smartphones to guide their in-store purchasing decisions, Google and other online companies have been under pressure to show advertisers how their online ads impact in-store behavior.

This roll out is a notable move for online to offline attribution generally and Google specifically. “Store Visits” is rolling out to eligible advertisers in the US over the next couple of weeks. Here’s what we know:

How It Works

Google determines a store visit based on user proximity to the advertiser’s location on Google Maps from users that have Location History activated on their Apple or Android smartphones.

As with phone call and cross-device conversions in ETC, store visits is an estimate. Google is using anonymized, aggregated user data and extrapolating it to the broader population.

The metric draws on search ad clicks across all devices — smartphones, tablets and desktops — and campaign types, including product listing ads and local inventory ads. Estimates are based on store visits within the last 30 days of an ad click.

Advertisers need to verify their business locations with Google and associate them with their campaigns to collect the store visit measurement.

Reporting is provided at the campaign level. Unless retailers break out campaigns by region or store location, store visit estimates won’t be shown at that level of detail.

This metric only estimates store visits, not purchases. Google is running a separate test to measure in-store transactions using retailer’s purchase data.

Why An Estimate?

“This feature has been carefully designed to keep data private and secure. We never provide anyone’s actual location to advertisers,” writes Surojit Chatterjee, Director of Product Management for Mobile Search Ads, in the announcement.

Google’s privacy thresholds mean that an individual’s actions won’t be attributed directly back to a search ad click.

Google has run into trouble over privacy violations, and data sampling and extrapolation is how it is choosing to give advertisers insights into user behavior while imposing relatively strict privacy restrictions on the data it shares with marketers.

Google has said previously in regards to cross-device conversions that the estimates are very conservative.

Who Is Eligible?

Because Google is relying on data sampling here, it needs a lot of inputs in order to get significant sample sizes. Qualifying advertisers are likely to be big box, multi-location retailers that have sufficient data. To find out if your business or clients qualify, contact your Google rep.

The program will likely expand beyond the small set it’s launching with if retailers show interest. The program could also grow beyond retail to other chains such as hotels, restaurants, movie theaters and car dealerships.

How Are Advertisers Using The Data?

Google cited two case studies of retailers that have been testing this metric. PetSmart’s estimated store visits data showed that 10 to 18 percent of clicks on search ads lead to a store visit. PetSmart increased its use of location extensions, which seems like something it should have been doing regardless of that new data. Office Depot has been using the metric to inform which products it features in its local inventory ads in Shopping campaigns.

Of course, a key reason for providing this metric is to prove the influence of search ads in driving offline sales and get advertisers to ramp their mobile investment in AdWords.

Whether advertisers embrace and trust this new metric will be significant for Google, particularly in light of Facebook’s own moves in this area with Atlas. For more analysis on this announcement, see Greg Sterling’s coverage on our sister site, Marketing Land.