Projecting paid search performance or PPC conversions as a function of impressions is problematic because any performance estimation, is ultimately a function of searcher behaviour.
Definition of an impression, taken from Google AdWords:
Each time someone searches on Google or the Google Network and your AdWords ad displays, it is counted as one “impression.” The “Impr” located on your reporting statistics shows the number of impressions for your ad.
However, when someone searches on Google using Google Instant, ad impressions are measured differently. When someone uses Google Instant, ad impressions are counted in these situations:
- The user begins to type a query on Google and clicks anywhere on the page (a search result, an ad, a spell correction, a related search).
- The user chooses a particular query by clicking the Search button, pressing Enter or selecting one of the predicted queries.
- The user stops typing, and the results are displayed for a minimum of three seconds.
Does an impression mean that someone actually sees the ad?
The answer here is no — just because the ad shows up on the search engine results page, does not mean that it has registered with the searcher.
If someone is using Google Instant and typing in queries, an impression may be counted when the searcher pauses for 3 seconds and then continues typing.
They may then have just glanced at the page and continued to search, potentially changing the search query to something not relevant to the product offering.
Three steps away from action
An impression is three steps away from action – first someone has to register your ad, look at it, decide to click on it and then interact with the landing page.
Without clicking and getting to the landing page, there is 0% chance of a conversion, because the person has to be physically on the landing page in order to take the desired action.
A percentage is only a number, so let us put a face on statistics
Conversion rate, is defined by conversions / clicks
Having a 1% conversion rate, means that for every hundred people that click on the ad, 1 will continue through and convert.
CTR (click through rate) is the ratio of impressions to clicks, defined by clicks / impressions
Having a 3% CTR means that for every hundred people who saw the ad, 3 will click on it
The above estimates hinge on a 100% confidence in the 1% or 3% estimate of a group of 100 people. (in a linear timeframe)
Statistically, that confidence is much lower and never 100%, so any percentage estimates will always vary, because you cannot predict human behaviour.
Using impressions to calculate conversions makes three assumptions:
1. Searcher will see the ad
2. Searcher will click on the ad
3. Searcher will convert on the landing page and take desired action
This is a statistically flawed argument because the confidence that someone will take all three actions is much lower than the assumed 1% conversion rate for someone who does click on the ad.
In fact, using the above averages, there is a 0.03% chance that someone who has not clicked the ad, will convert. Thus, the projections will be unrealistically inflated with no statistical probability that it is even possible to reach the projected conversions when calculated as a function of impressions.
There’s a great deal of advice out there about