When a line item starts its campaign, DFP creates an initial delivery plan that it then modifies based on changes in the system over time.
The ad server uses the following step-by-step methodology to create a plan to pace the delivery of your line items. Each step modifies the number of impressions the system sets for its daily and hourly delivery goals. Click through each step to read more about what calculations are made to serve your line items.1. Initial calculations
After a an even delivery line item is booked, DFP calculates how many impressions it should attempt to serve for the line item per day. This calculation is simply the number of impressions booked for the line item divided by the number of days in the line item's campaign. These goals represent truly even delivery, as shown in Chart 1 below.
Imagine a week-long campaign with 7,000 scheduled impressions. Perfectly even delivery would result in 1,000 impressions being served each day of the week.
Next, DFP looks at forecasting data to understand how traffic to the targeted inventory will look over the life of the line item. DFP then attempts to serve the line item in proportion to the total traffic to your inventory. For example, if traffic to the targeted inventory is very high during the evenings and on weekends, DFP will serve more impressions at these times and less during slower times. These forecasting hints to the ad server allow DFP to deliver impressions smoothly relative to the natural traffic fluctuations on your site.
Basically DFP takes the forecasted eligible traffic for a line item and determines what percentage that line item should serve to, similar to a share-of-voice ad. For example, if a line item has a one million impression goal, and we forecast three million eligible impressions, then the line item will serve one-third of the time, regardless of the time of day or day of the week. This improves advertiser satisfaction and prevents calls from advertisers asking, “Where’s my ad?”
The graph below illustrates this point.
Forecasted delivery uses past impression data to predict trends for the future to allow ad serving to take advantage of traffic patterns to your websites. For example, imagine a website whose traffic is highest between 4:00 PM and 1:00 AM each day. The ad server will serve less impressions during the low traffic hours in the early morning and will be more aggressive during the high-traffic evening hours.
DFP also adds a “boost” factor starting five days before the end of the campaign that increases in magnitude as the line item approaches the end date. Line items that are behind schedule will get a stronger boost factor than on schedule line items. This is to ensure that line items are able to deliver on time, even in the event of last minute traffic drops. We differentiate between on schedule and behind schedule line items in order to allow on schedule line items to finish as close to the end of the delivery window as possible.
Changes over time
Over time DFP has to adjust the goals for your line item based on the actual number of impressions it has served. The originally calculated goals, as outlined above, are only valid when the server delivers exactly to the goal every hour. Click through each step below to see which calculations are updated and modified throughout delivery.1. Satisfaction Index
DFP calculates a satisfaction index (SI) to measure how far ahead or behind schedule a line item is. Specifically SI measures how well a line item has performed over the last 24 hours, relative to the goals set for it by the ad server. The SI is then used during the line item selection process so that line items that are further behind are selected more frequently. If you are not generally familiar with SI and how it is used in the line item selection process, please see the ad selection whitepaper.
Rather than selecting a line item directly based on SI, DFP instead sets a different threshold for each line item. The SI for the line item must exceed this threshold in order for the ad server to select it amongst all of the line items that accepted the ad server's request. This threshold is also calculated once an hour and is controlled by the Proportional Integral Derivative (PID) controller.
The PID controller attempts to make a line item react more gradually to changes in its environment such as unexpected changes in your site’s traffic levels. Instead of dramatically decreasing the threshold every time the SI starts to fall, the PID controller instead takes a more gradual approach. When setting the threshold it considers both how far away the line item is from a perfect SI and how quickly the SI of the line item is changing. The net effect is that your line items will deliver more evenly than they did in the DART ad server.
Since the PID controller’s adjustments are gradual, it will take your line item several hours to fully correct if it gets ahead or behind. Although this may seem odd in the short term, it significantly improves delivery over the life of the line item.
DFP regularly recalculates the daily and hourly delivery goals. This follows the same process outlined for a new line item above, but uses updated information. Impressions are allocated based on the total remaining impressions and the total remaining time in the line item's campaign. For line items that have been live less than 24 hours, these delivery goals are updated every hour. After the first day the delivery goals are updated every eight hours. The goals are updated more frequently for new line items to allow the server to calibrate itself based on how the new line item is delivering.
The ad server also takes hints from the forecasting system when determining how many times to serve a line item in a given time window. Once a day the ad server pulls in updates from the forecasting system, then serves each line item proportionally to the expected traffic on an hour by hour basis.
These forecasting adjustments are not reflected in your line item’s progress indicator. Before making any adjustments to a line item, check to see if the line item is forecasted to deliver in full. For example, if the ad server knows you have higher traffic on the weekend, the progress indicator may show that a line item may be behind schedule after delivering for a few weekdays; however, this is not a problem because the server knows it can serve the line item more on the weekend. Always make sure to check the forecast for a line item before making any delivery changes.