Compound tracking is a technique in which one single tracking asset (any piece of content associated with an ad or the page on which an ad appears that is designated to serve as the ―trigger by which the ad is counted , it can be 1×1 pixel image , 302-redirect(found),a javascript code or the ad itself) is allowed to register impressions for multiple ads that are in separate locations on the page, as long as reasonable precautions are taken i.e. to ensure that all ads that are loaded and recorded prior to the tracking asset being called which means the ad should load before and the count is made only after the final ad has been rendered or loaded into the page.
For example, an ad group may be counted if reasonable assurance exists that all grouped ads load prior to counting, such as through the placement of the tracking asset at the end of the HTML string.