Since we introduced product ratings, we’ve been working on new ways to help you gather feedback about your products and drive more qualified traffic to your site. Today, we’re making product ratings available in all markets where Google Shopping is present.
Product ratings appear in the form of stars and review counts on Shopping ads. This 5-star rating system represents aggregated review data for the product, compiled from multiple sources including merchants, third-party aggregators, editorial sites and users.
How to enable product ratings on Shopping ads
If you have a Merchant Center account for your store, you’ve already taken the first step towards displaying Product Ratings. Once you sign up and satisfy the program requirements, you can begin uploading Product Ratings feeds to your Merchant Center account. Alternatively, you may want to work with one of our approved third-party aggregators.
Additionally, you can now collect reviews about the products you sell through Google Customer Reviews. Whenever your shoppers complete a purchase, we’ll ask them about their shopping experience with your store, as well as the product they purchased. The reviews collected about their shopping experience will contribute to your seller rating and, the product reviews collected can make you eligible to show product ratings. To learn more about how to use Google Customer Reviews to collect product reviews, see here.
Source: Official Google Webmasters Blog