Google announced this week that it will be rolling out updates (over the next few days) to ad group-level impression share metrics and revising campaign-level impression share metrics for the search and display networks.
Impression share is the percentage of impressions received divided by the estimated number of impressions an advertisers was eligible to receive. Advertisers will now see the share of impressions lost due to their Ad Rank and the Exact Match impression share which is the percentage of impresses an advertisers received (for the search network only) that exactly matched the keyword divided by the estimated number of exact match impressions the advertiser was eligible to receive.
Google will be updating all campaign-level impression share metrics all the way back to May of 2011. Google indicated that it will be updating impression share metrics once per day so don’t expect to see accurate impression shar metrics for the current day.