Guaranteed delivery pacing

Guaranteed Delivery line items are designed to deliver their budgets in full and evenly across their flight dates. How guaranteed line items pace determines how well they meet the requirements of those budgets.

This page describes the pacing algorithm that governs how Guaranteed Delivery line items meet their budgets. For simplicity, this page describes even distribution of the budget across each day of the flight. See Forecast-shaped pacing for more information on advanced pacing capabilities.

The ideal delivery curve

For a guaranteed line item with a delivery goal of 1 million impressions over a 60-day flight that is set to the default pacing value of 105%, the ideal delivery curve looks like this:

To achieve the ideal delivery curve, we have to control the delivery rate across a distributed system of bidders over the entire length of the flight. Broadly speaking, there are two ways to control the delivery rate:

1. Raising or lowering the bid price
2. Deciding whether to bid or not

The challenge

Due to physical constraints (such as how fast information propagates across networks), it's impossible to adjust bid prices and bid/no-bid decisions in real-time across our distributed system of bidders to precisely match bid request rates and win rates. Instead, information can only be propagated across systems at slower-than-realtime speeds.

Furthermore, even if it were possible to adjust bid decisions in real-time, the request rates and win rates across a slice of inventory are often extremely volatile, as shown in the graphs below. Due to the fact that budgets are distributed across our many bidders and any real-time calculations have to occur locally at the bidder level, were we to attempt to adjust bid decisioning in response to this information, each bidder would have either a very outdated estimate of the request and win rate (due to slower-than-real-time updates), or a very noisy one (thanks to the volatility of the graphs).

To give you some idea of the volatility involved, here are the bid requests per minute across one randomly selected slice of inventory on the Microsoft Advertising platform over a 24-hour period in early 2015:

Here is the win rate of a $0.10 CPM bid for a similar randomly selected slice of inventory graphed over a 24-hour period in early 2015:

Our solution

Given these challenges, and the requirement that we maintain the even delivery rate required by sellers, we forego attempting to update bidders in real-time and instead dynamically estimate the request rates and win rates algorithmically, based on a statistical analysis of sample data. Specifically, we use a Logistic regression, which is a fancy way of saying that we use a mathematical model to guess the probability of an event given some other data.

In addition to mathematical modeling of request rates and win rates, we do a few other things to make sure that guaranteed line items meet publisher commitments:

• Set the pCPM bid price slightly higher than strictly necessary to ensure robust win rates.
• Cap the delivery rate to avoid over-delivery by having individual bidders go to sleep (i.e., stop bidding) when ahead of even delivery by a certain percentage.
• Set the daily budget goal at 105% of even lifetime delivery by default to ensure goals are met.
• Ensure that when guaranteed line items compete for the same impression, we select one based on a weighted average of their target win rates. In other words, most of the time, we choose the guaranteed line item that is further from meeting its delivery goals to serve on eligible impressions.

For more information about pCPM and how competition between guaranteed line items works, see Guaranteed Delivery Auction Mechanics.

Behavior at the start of the flight

At the start of the flight, guaranteed line items will attempt to "front-load" slightly to ensure that budgets are delivered (assuming that the line item pacing is set to the default of 105%). This, combined with the settings described above, should cause the delivery curve to look like the graph in The Ideal Delivery Curve section.

Behavior at the end of the campaign

Currently there is no special behavior that takes place during the final hours of the line item's flight. Line items continue to pace according to the algorithm described here until the flight ends.

Timing of budget updates

Unlike some other systems that only recalculate pacing goals every hour or eight hours, the pCPM values that allow guaranteed line items to compete with RTB bids are recalculated locally by the bidder every few minutes. This is supplemented by the global budget update that goes out to all bidders, which happens approximately every 15 minutes.

Example

When a line item is underdelivering, or if a budget is increased mid-flight, the line item will enter a catch-up state whereby we re-allocate underdelivered impressions. Guaranteed line items can be configured (see Create a Guaranteed Delivery Line Item for more details) either Evenly or ASAP underspend catch up which determines the line item’s delivery behavior. Note that in either underspend case, lifetime pacing and/or day-of week forecast-shaped pacing will continue to adjust daily allocation accordingly.

Even underspend catch-up

When a line item is configured with Evenly Underspend Catch-Up, any unspent daily budget is evenly redistributed across the remaining days of the flight.

For example, if a guaranteed line item falls short by 100,000 impressions with five days remaining in the flight and has a daily delivery goal of 20,000 impressions, the system will attempt to serve 40,000 impressions during the current 24-hour period (the original 20,000 daily goal plus 20,000 catch-up impressions for the remainder of the flight).

Consider another example: 7-day campaign with a total budget of 700,000 impressions, targeting 100,000 impressions per day:

• Day 1: The line item delivers 100,000 impressions—right on target.
• Day 2: The line item is paused early in the day.
• Days 3–5: The line item remains paused and does not serve.
• Day 6: The line item is re-enabled. It is now behind by approximately 400,000 impressions. To begin catching up, it attempts to serve 300,000 impressions: 100,000 for the current day, plus 200,000 to start recovering the shortfall (evenly split over the remaining two days).
• Day 7: If 300,000 impressions were successfully delivered on Day 6, it will again aim for 300,000 on Day 7—100,000 for the day’s goal and 200,000 to complete the catch-up. If Day 6 fell short, the line item will adjust and try to make up the remaining deficit in addition to meeting the Day 7 goal.

ASAP (Default) Underspend Catch-Up

When a line item is set to ASAP Underspend Catch-Up, it attempts to recover from underdelivery by making up the shortfall within the next 24-hour period.

For example, if a guaranteed line item is trailing by 100,000 impressions and its daily delivery goal is 20,000 impressions, it will aim to deliver 120,000 impressions in the current 24-hour window, meeting the daily target while also compensating for the backlog.

Consider another example: 7-day campaign with a total budget of 700,000 impressions, targeting 100,000 impressions per day:

• Day 1: The line item delivers 100,000 impressions—right on target.
• Day 2: The line item is paused early in the day.
• Days 3–5: The line item remains paused and does not serve.
• Day 6: The line item is re-enabled. It is now behind by approximately 400,000 impressions. To begin catching up, it attempts to serve 500,000 impressions on Day 6:
  • 400,000 to recover the missed delivery
  • 100,000 to meet the current day’s goal
• Day 7:
  • If the full catch-up was successful on Day 6, it resumes normal delivery with 100,000 impressions.
  • If not, it continues catch-up by attempting to deliver the remaining deficit plus the 100,000 impressions scheduled for Day 7.

How the daily budgeting process works

For any given line item, the daily budgeting process will run once per day at midnight in the timezone of the line item. The basic steps of this process are:

1. Calculate the delivery amount needed to catch up based on the line item catch up configuration (evenly vs ASAP).
2. Calculate the delivery amount needed to catch up in full by the end of the coming day.
3. Calculate the historical rate of underdelivery, looking back over the past 4 days.
4. Divide the delivery amount calculated in the first step (#1 above) by the average underdelivery rate (#2 above) to account for persistent underdelivery. This value is used as our daily delivery goal.

Related topics

• Guaranteed Delivery
• Guaranteed Delivery Auction Mechanics
• Using Multiple Campaigns with a Guaranteed Line Item
• Create a Guaranteed Delivery Line Item
• Forecast Shaped Pacing