Impression statistics can help you evaluate the effectiveness of your ads. You can view ad campaign reports after your ads start displaying. To view this data, go to the Reports Wizard.
For example, you can see what ad sizes are served most frequently. In the Cross sections block, select Ad № and Image size, in the Columns block, select the Impressions value.
For some parameters (audience reach, average impression frequency), the system uses a mathematical-statistics method to calculate figures based on a selection of users. This lets you quickly get the parameter value for a large amount of data. Therefore, the reach displayed in statistics may not differ significantly from the true value.
Read more information about campaign statistics.