Google Adwords Impression Share Metric Explained
Adwords introduced a new reporting metric called Impression Share in April which is only available at both the account and the campaign levels.
Impression Share is the share of all impressions an ad acquired against other ads that are competing for the impressions. This is how the metric works on both the account and campaign levels.
For example, let’s say Campaign A and Campaign B both belong in the same account. Out of the ten potential impressions, Campaign A shows twice, and Campaign B shows five times. A competitor takes the remaining three impressions . A campaign-level report will show an impression share of 40% for Campaign A (2/(2+3)) and 63% for Campaign B (5/(5+3)).
Let’s take the same example from above. An account-level report would show an impression share of 70% ((2+5)/(7+3)) for the entire account.
Vinny Lingham also explained the effects on Search Affiliates and Search Agencies if Impression Share is being measured.
[...] month, new metric like the Impression Share was made available on the Campaign and Account [...]
[...] are able to easily review how their marketing budgets are distributed. On top of that, the impression share metrics were made available in some of the higher level [...]