How to investigate discrepancies – DoubleClick

The best practice is to always traffic DFA tags using the "DoubleClick tag" creative type. If you have trafficked, your DFA tags as a DoubleClick tag creative type and are experiencing a discrepancy of greater than 2%.

Normally, DFP records an impression as soon as the ad server determines which creative to send. When the DoubleClick tag creative type is chosen, however, DFP does not record the impression until the line item has been sent and an impression has been recorded by DFA. As a result, this process reduces discrepancies between DFP and DFA, typically to within 2%. The DoubleClick tag creative type will also eliminate the need to add click tracking macros.

If a DoubleClick tag is booked as a custom or third-party creative, this enhanced functionality is lost; DFP will treat the DoubleClick tag as a third-party tag. It will not report clicks without the DFP click macro, and you will see a larger discrepancy in delivery numbers (as you would with any third-party ad server). If you have trafficked DoubleClick tag as a custom or third-party creative type and are experiencing a reporting discrepancy between DFP and DoubleClick for Advertisers (DFA), please see the “Third-party discrepancies” article. Otherwise, explore the following possibilities.

Discrepancies may result from:

Invalid activity filtering: DFP and DFA differ slightly in filtration methodologies, which can result in discrepancies in instances when frequent invalid activities occur.

Things to check:

• Has the correct DoubleClick tag been trafficked? Like DFP, DFA has many tag types, including one that is specifically intended for use in DFP.
• Are you comparing the correct objects between DFA and DFP? Which DFA ads are included in the placement for the tag you’ve trafficked in DFP?
• Have you or the advertiser made any changes during the time in question (e.g., reassigning or modifying creatives)?
• Are you looking at daily data or aggregated results? Break out the data by day and line item.
• Does the advertiser code load properly? Perform live tests on web pages where the line item should deliver to verify that the correct ad tag calls are being made.

Source: Google Support