Google are finally releasing impression share at an ad group level, rolling out as we speak across accounts globally. To facilitate this much anticipated release, Google are not only having to re-work the algorithm that works it out but also deleting pre-May 2011 data (suggesting they probably only keep detailed back end account data for around 6 months, hence can’t recalculate before this time).
Why a recalculated impression share?
At campaign level, impression share took available impressions across all ad groups. Now the data is more granular, the system needs to consider individual ad group data. Google have suggested the overall campaign level impression share may change after the recalculation. This may be due to poor accuracy in the old algorithm.
Furthermore, ad group impression share is more challenging as budgets are not set at this level. To go back and look at impression share more closely, Google will need to account for campaign budget level and if certain ad groups took larger shares of the daily budget.