Compare Ad Manager Historical reports and line item forecasts

Why Ad Manager "Check inventory" forecasts differ from Historical reports

In Google Ad Manager, there are two main ways to predict availability:

  • Line item forecasts appear when you click Check inventory when creating or viewing a line item. They provide a forecast focused on a single line item. This can help you maximize the number of impressions for that particular line item and targeting.
  • Ad Manager Historical reports are a report type that displays historical data related to ad requests flowing through the Ad Manager network, including linked Ad Exchange web properties. The data includes clicks, revenue, click-through rate, and total impressions for dates in the past.

Possible causes of discrepancies

While Ad Manager Historical reports provide past information about inventory impressions, "Check inventory" forecasts future inventory impressions based on past data. There will always be a certain level of discrepancy when making future-looking forecasts. However, a few areas to explore when observing significant discrepancies are highlighted below. Note that there might be additional differences in behavior not covered here.

  • The values forecast by "Check inventory" don't, by default, account for the impression volumes of special ad units within targeted placements. Special ad units will need to be explicitly targeted by the line item that is being considered for the forecast. Ad Manager Historical reporting, however, does include impression volumes for special ad units.
  • "Check inventory" forecasts how many impressions a line item can safely reserve with the applied targeting criteria. One of the assumptions that decides the margin of safety is that no single creative of a particular size will be served more than once by the line item on a page. For example, a creative of size 300x250 would be assumed to serve only into one slot in the page (Default Creative Count = 1). However, some pages might have multiple slots of the same size that may have the same line item serving the creative many times.

    Ad Manager Historical reports capture these instances of multiple serves and impressions. However, to accurately forecast for pages with multiple slots, the "Check inventory" forecast increases the creative count estimate within the forecast to account for these instances.
     
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