An impression is a metric that counts when content or an advertisement is seen by a user. It can be seen by the same user multiple times, resulting in multiple impressions for that one user. It is not to be confused with reach which is how many unique users have seen the content or advertisement.
Why would I need this?
A popular method of purchasing ad space is by the number of impressions the format generates. Advertising formats tend to charge per thousand impressions, on a cost per mille (CPM) basis.
Advertisers need to know how many impressions a CPM advert generates to understand campaign cost.
More broadly, impressions are important because they provide a representation of how many people are seeing ads within a particular channel, and quickly ascertain how far an advertising channel reaches.
Knowing the impression count also helps marketers to generate other marketing metrics too, such as click-through rates (CTR). These metrics are used to calculate a campaign’s effectiveness, but require an accurate impression count to be successfully measured.
How does it work?
Marketers and advertisers have debated what constitutes an impression as they can be easily skewed by a single user viewing the same ad repeatedly or users not actually seeing the ad on the page for example. This makes advertisers sceptical about impression figures. Most advertisers and publishers decide how impressions are accounted for before purchasing.
There are two types of impressions and they differ in accuracy. Served impressions are counted when an advert is accessed by a user whereas a viewable impression excludes the cases where the ad was not actually seen, making it more accurate. This can include ad-blocking software, screen resolutions that are too small, users scrolling before the ad was loaded, broken plug-ins, minimised browser windows, and more.
Viewable impressions provide advertisers with more accurate information on the number of actual impressions and the data collected can suggest improvements to optimise greater rates of content delivery.
Real world examples