Determine impressions to book

Forecasting helps you understand how many impressions are available for a prospective line item. The number of impressions available depends on how much traffic your website or app gets—that is, how many people visit pages of your website or use your app.

To check available inventory:

1. Run a forecast for the prospective line item. Targeting and other delivery settings affect the number of available impressions.
2. Examine the results on the first page of the forecast for guidance on how many impressions to book.
3. Analyze and adjust the effect of booking the prospective line item by viewing contending line items.

The number of impressions you can book for a prospective line item depends on how willing you are to displace delivery for contending line items of the same or lower priority. Booking more impressions than are available means that these line items could underdeliver. 

While cost is not a criteria considered in forecasts, you can view the revenue impact of contending line items. You may be willing to displace delivery for contending line items if the prospective line item is predicted to bring in more revenue.

How forecasting accounts for trafficking criteria

Forecasting accounts for most targeting and other delivery settings when determining available inventory. Below are some considerations for how forecasting accounts for these details.

Delivery setting	Consideration
Frequency capping	Frequency capping is considered, but monthly or lifetime frequency caps are not captured as part of the data. Consequently, in cases with monthly and lifetime frequency caps, forecasting can overpredict availability. For shorter capping windows, forecasting assumes that every future user has their full allowance of impressions still available, so it will be most accurate before the start of a campaign, and may overpredict later.
Priority	Forecasting looks at priority to determine the Reserved by same-or-lower priority line items number--the number of impressions a line item can get if you’re willing to take impressions from same- or lower-priority line items. Priority is also taken into account when calculating Impressions likely to deliver.
Count and creative details	Some line items require more than one creative for the same inventory size. Entering a value in the Count field of creative details can account for multiple creatives of the same size, and a count allows for better forecasting. Without a count, forecasting assumes only one creative per size and could fail to provide an accurate forecast. For example, suppose there's a line item that requires two 300x250 creatives. The targeted inventory is a webpage with two ad slots of that size. Without accounting for count, forecasting counts one visit to the page as one impression for a single ad slot of 300x250. However, in reality, two creatives are intended to serve to each ad slot, so the forecast should really predict 2 impressions.
Deliver impressions	Ad Manager simulates all line items as evenly paced, regardless of their actual "Deliver impressions" setting.
Roadblock ads	When you run a forecast on roadblock ads, Ad Manager records which impressions occurred together when a page is viewed--this is called page awareness. And forecasting takes into account the type of roadblock you're interested in booking, those set up to work with all of the creatives in the line item, or those that work with master/companion creative sets. For example, a line item containing one creative that’s 728x90 and one that’s 300x250 has these forecasted page views in the following distribution: Page A: the 728x90 creative serves 2 times without the 300x250. Page B: the 300x250 creative serves 1 time without the 728x90. Page C: both creatives serve together (the roadblock) 4 times. In this scenario, the forecasting model lets you know that 4 roadblock impressions are available, because it's aware that only Page C can serve the roadblock with both creatives.