Google provides a precise definition of an impression. You’d think there’s nothing left to define — an impression is an impression, everyone can see that — but it turns out that’s not quite the case.
(See the definition of an impression in the Google Ads help article.)
Shopping campaigns
Impression and CTR metrics in Shopping ads are not comparable with their counterparts in search campaigns. That’s because only one ad for a given site can appear in the search results, whereas several Shopping ads can show at once.
Each product’s appearance is counted as an impression, and CTR is calculated against all impressions, which in a sense can “deflate” the CTR of Shopping ads.
Even if every user searching for a given term clicked our ad, in search ads that would mean a CTR of 100%, while in Shopping campaigns it would additionally depend on how many products are displayed. If there were, say, 3 products, the maximum possible CTR would be 33%.
If we raise bids (manually or indirectly through a bidding strategy), we may find that impressions and clicks grow while CTR falls. The reason: our product takes higher positions and gets more exposure, so the chance that a user clicks our ad increases — but the number of products shown to the user increases as well, and with it the number of impressions in the denominator of the CTR formula (clicks divided by impressions).
Unfortunately, Google does not (yet) report unique ad impressions, nor the average number of products displayed in Shopping results. However, to count impression share correctly, Google does account for multiple impressions: when calculating the impression share of Shopping ads, it only counts impressions of the first product (the one in the highest position).
It’s worth keeping this in mind when comparing the CTR and impressions of Shopping ads with search ads. The relatively low CTR of Shopping ads doesn’t have to mean they “get clicked less” — it may simply reflect the larger number of products displayed to a given user.
Performance Max campaigns
If your Performance Max campaign uses a product feed, several products appearing on the search results page will be counted as… a single impression (simultaneous impressions of multiple products are deduplicated). So it works differently than in Shopping campaigns.
There is a way to see product impressions before deduplication. When you browse the Products tab of a Performance Max campaign, or add the product name, product ID, or Merchant Center ID dimension to your reports, you’ll see non-deduplicated impressions, because this data is pulled from Merchant Center. This can make it look like the products generate more impressions than the entire campaign.
It’s not a bug, it’s a feature.
