Compare Ad Manager Historical reports and line item forecasts

Why "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 while 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 information about past 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, some  areas to explore when observing significant discrepancies include: 

  • By default, "Check inventory" doesn't include impressions from special ad units within targeted placements in forecasts. Special ad units need to be explicitly targeted by the line item the forecast applies to.

    Historical reports do include impressions from 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 this 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 and the line item may serve the creative many times.

    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.

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