What is Impression Share?
Impression Share (IS) is the percentage of impressions you received divided by the estimated number of impressions you were eligible to receive. Eligibility is based on your ad’s targeting settings, approval statuses, bids, and Quality Score.
Changes to Impression Share Metrics in Google AdWords
Up until recently impression share metrics were only available at campaign level in AdWords. At the end of January 2012 Google rolled out Ad Group Level Impression Share for both the Search and Display Networks. This new feature enables advertisers to better understand how individual ad groups are performing as well as identify ad groups which are not capturing the majority of available impressions and therefore require more funds.
With the introduction of ad group level Impression Share (IS) there are three new columns that can be added to the ad groups tab within AdWords. These are: IS, Lost IS (rank) and Exact match IS. Lost IS (rank) shows the percentage of impressions lost due to ad rank and Exact match IS the percentage of impressions you received for searches that exactly matched your keyword divided by the estimated number of exact match impressions you were eligible to receive.
In addition to bringing the Impression Share (IS) data to ad group level Google have also updated their algorithms it uses to calculate IS metrics. All existing campaign-level information has been updated back to May 2011 which means that you will no longer be able to see IS data prior to this time. The new updated algorithm will also make for a more accurate campaign IS since it is now being updated once a day around noon Pacific Time, so depending on when you run the report, the impression share data may not reflect the reports for the current day.