Handling Advertisements of Unknown Quality in Search Advertising Sandeep Pandey Christopher Olston (CMU and Yahoo! Research)

Sponsored Search  How does it work? Search engine displays ads next to search results Advertisers pay search engine per click  Who benefits from it? Main source of funding for search engines Information flow from advertisers to users

Sponsored Search  Click-through-rate (CTR): given an ad and a query, CTR = probability that the ad receives a click  Optimal policy to maximize search engine’s revenue: display ads of highest (CTR x bid) value Search query results Sponsored search results

Challenges in Sponsored Search  Problem: CTRs initially unknown estimating CTRs requires going around the circle  Exploration/Exploitation Tradeoff: explore ads to estimate CTRs exploit known high-CTR ads to maximize revenue refine CTR estimates record clicks show ads earn revenue

The Advertisement Problem  Problem: Advertiser A i submits ad a i,j for Query phrase Q j User clicks on a ij -> A i pays b ij (the “bid value”) Queries arrive one after another Select ads to show for each query, in an online fashion  Constraints: Show at most C ads per query Advertisers have daily budgets: A i pays at most d i  Goal: Maximize search engine’s revenue Advertisers Query phrases a 1,1 A1A1 Q1Q1 a 2,1 a 1,3 A2A2 A3A3 Q2Q2 Q3Q3 a 3,2 d1d1 d2d2 d3d3 BudgetsAds

Our Approach  Unbudgeted Advertisement Problem Isomorphic to multi-armed bandit problem  Budgeted Advertisement Problem Similar to bandit problem, but with additional budget constraints that span arms Introduce Budgeted Multi-armed Multi-bandit problem (BMMP)

Unbudgeted Advertisement Problem as Multi-armed Bandit Problem  Bandit: Classical example of online learning under the explore/exploit tradeoff K arms. Arm i has an associated reward r i and unknown payoff probability p i Pull C arms at each time instant to maximize the reward accrued over time p1p1 p2p2 p3p3  Isomorphism: query phrase bandit instance; ads arms; CTR payoff probability; bid reward

Policy for Unbudgeted Problem  Policy “MIX” ( adopted from [Auer et. al. ML’02] )  When query phrase Q j arrives Compute the priority p i,j of each ad a i,j where p i,j = (e i,j + sqrt(2 ln n j / n i,j )). b i,j  e i,j is the MLE of the CTR value of a i,j  b i,j is the price or bid value of ad a i,j  n i,j : # times ad a i,j has been shown in the past  n j : # times query Q j has been answered Display the C highest-priority ads

Budgeted Multi-armed Multi-Bandit problem (BMMP)  Finite set of bandit instances; each instance has a finite number of arms  Each arm has an associated type  Each type T i has budget d i Upper limit on the total amount of reward that can be generated by the arms of type T i  An external actor invokes a bandit instance at each time instant the policy must choose C arms of the invoked instance

Meta Policy for BMMP  Input: BMMP instance and policy POL for the conventional multi-armed bandit problem  Output: The following Policy BPOL Run POL in parallel for each bandit instance B i Whenever B i is invoked:  Discard arm(s) with depleted budget  If one or more arms was discarded, restart POL i  Let POL i decide which of the remaining arms to activate

Performance Guarantee of BPOL  OPT = algorithm that knows in advance: 1.Full sequence of bandit invocations 2.Payoff probabilities  Claim: bpol(N) >= opt(N)/2 – O(f(N)) bpol(N): total expcted reward of BPOL policy after N bandit invocations opt(N): total expected reward of OPT f(N): regret of POL after N invocations of the regular bandit problem

Proof of Performance Guarantee  Divide the time instants into 3 categories: 1 : BPOL chooses an arm of higher expected reward than OPT  opt 1 (N) <= bpol 1 (N) 2 : BPOL chooses an arm of lower expected reward because OPT’s arm has run out of budget  opt 2 (N) <= bpol 2 (N) + (#types. max reward) 3 : otherwise  opt 3 (N) = O(f(N))  Claim (implies from the above bounds) opt(N) <= bpol(N) + bpol(N) + O(1) + O(f(N)) bpol(N) >= opt(N)/2 – O(f(n))

Advertisement Policies  BMIX : Output of our generic BPOL policy when given MIX as input  BMIX-E : Replace sqrt(2 ln n j / n i,j ) in priority p i,j by sqrt(min(0.25, V(n i,j,n j )). ln n j / n i,j ), where V(n i,j,n j ) = e i,j.(1-e i,j ). sqrt(2 ln n j / n i,j ) Suggested in Auer. et. al. ML’02. Purpose: Aggressive exploitation  BMIX-T : Replace b i,j in priority p i,j by b i,j. throttle(d i ‘), throttle(d i ‘) = 1-e^(- d i ‘/d i ) where d i ‘ is the remaining budget of advertiser A i Suggested in Mehta et. al. FOCS’05 Purpose: Delay the depletion of advertisers’ budgets  BMIX-ET: with both E and T modifications

Experiments  Simulations over real data  Data: 85,000 query phrases from Yahoo! query log Yahoo! ads with daily budget constraints CTRs drawn from Yahoo!’s CTR distribution Simulated user clicks using the CTR values  Time horizon = multiple days Policies carried over the CTR estimates from one day to the next

Results  GREEDY : select ads with highest current reward estimate (e i,j. b i,j ) Does not explore. Only exploits. *Revenue values scaled for confidentiality reasons

Conclusion  Search advertisement problem Exploration/exploitation tradeoff Model as multi-armed bandit  Introduced new Bandit variant Budgeted multi-armed multi-bandit problem (BMMP) New policy for BMMP with performance guarantee  In paper: Variable set of ads (ads come and go) Prior CTR estimates