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Two impression metrics track how prominent your ads are: impression share (IS) and absolute top impression share (ATIS)

Impression share

Impression share (IS) is the percentage of impressions that your ads receive compared to the total number of impressions that your ads could get.

Impression share = impressions / total eligible impressions

Eligible impressions are estimated using many factors, including targeting settings, approval statuses, and quality. Impression share data is available for campaigns, ad groups, product groups (for Shopping campaigns), and keywords.

Impression share is a good way to understand whether your ads might reach more people if you increase your bid or budget.

Absolute top impression share

Absolute top impression share (ATIS) is the percentage of your Shopping ad impressions that are shown in the most prominent Shopping position. There’s only one “absolute top impression” per auction so your absolute top impression share is an important indicator of your overall prominence.

Absolute top impression share = absolute top impressions / total eligible top impressions

Absolute top impression share can help you understand whether your products might appear more prominently in Shopping ad results if you increase your bid or budget. ATIS is available for campaigns, ad groups, and product groups (for Shopping campaigns).

How it works overall

To estimate impression share, Google looks at the ad auctions over the course of the day, and uses internal data such as quality for you and other participants in the same ad auctions. 

Impression share includes all auctions where your ad showed, and all auctions where your ad is competitive enough to show. For example, it could include auctions where your ad could show at twice its current bid, but could exclude auctions where your ad is estimated to need a 1,000% bid increase in order to appear. 

Keep in mind, impression share is based on an estimate of when your ad was competitive in the auction. Small fluctuations over time don’t necessarily indicate that action is needed. Changes to your bids, quality, or Google's ad systems may change the set of auctions in which the system estimates you were competitive.

How it works for Shopping campaigns

Impression share for Shopping campaigns is calculated similarly to text ads. However, this metric considers that multiple Shopping ads from the same advertiser can show at the same time. 

To avoid double counting when multiple Shopping ads from the same advertiser are in the same auction, AdWords assigns the impression and the impression opportunity to the highest ranked Shopping ad from that advertiser. When that impression is in the most prominent possible position, it’s also counted in absolute top impression share.

Absolute top impression share includes all auctions where your ad is shown in the most prominent position divided by all auctions where your ad could have been shown in the most prominent position.