Impression Share may be one of the most useful and least utilized metrics in search marketing – exact match impression share, that is.
Utilizable for the search network only, exact match impression share tells the advertiser what percentage of the possible impressions for their keywords (taking match types into account) the ads are showing up on. A 100% impression share means the advertiser’s ads are showing up for every search on the keywords in the campaign. A 25% impression share means the advertiser’s ads are only showing up 25% of the time.
The two main reasons an impression share might not be 100% are budget and rank. While there are metrics to these effects as well, it is important to consider them in conjunction with the exact match impression share.
To view impression share, add exact match impression share under columns (it’s listed as a competitive metric in the new interface) on either the campaigns or ad group tabs.