utilizing query change for session search (sigir 2013)
TRANSCRIPT
U"lizing Query Change for Session Search
Dongyi Guan, Sicong Zhang, Grace Hui Yang Department of Computer Science
Georgetown University
Speaker: Grace Hui Yang
SIGIR 2013 @ Dublin
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Introduc"on • Session search – performs document retrieval for a session of search queries
• TREC Session tracks (2010-‐2012) – Given a series of queries {q1,q2,…,qn}, top 10 retrieval results {D1, … Di-‐1 } for q1 to qi-‐1, and click informa"on
– The task is to retrieve a list of documents for the current/last query, qn
• Relevance judgment is made based on how relevant the documents are for qn, and how relevant they are for informa"on needs for the en"re session (in topic descrip"on)
– no need to segment the sessions
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In a session, queries change constantly
1.pocono mountains pennsylvania 2.pocono mountains pennsylvania hotels 3.pocono mountains pennsylvania things to do 4.pocono mountains pennsylvania hotels 5.pocono mountains camelbeach 6.pocono mountains camelbeach hotel 7.pocono mountains chateau resort 8.pocono mountains chateau resort a^rac"ons 9.pocono mountains chateau resort ge`ng to 10.chateau resort ge`ng to 11.pocono mountains chateau resort direc"ons
TREC 2012 Session 6
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Informa"on needs: You are planning a winter vaca"on to the Pocono Mountains region in Pennsylvania in the US. Where will you stay? What will you do while there? How will you get there?
Outline
• Introduc"on to session search • What is Query Change and Why it is important for session search?
• Where do these query changes come from? (Why the user makes those query changes?)
• How to model query changes and the dynamics in session to help retrieval?
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Defini"on of Query Change
• We define query change as the syntac"c edi"ng changes between two adjacent queries:
• includes – , added terms – , removed terms
• The unchanged/shared terms are called: – , theme term
1−−=Δ iii qqq
iqΔ
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iqΔ+iqΔ
iqΔ−
themeq
First take-‐home message: Query Change is an Important Form of Feedback
• It is probably the strongest signal that a user likes or dislikes the retrieval results for previous queries • Much stronger than
– clicking a document – reading a document for > 30 seconds (SAT clicks) – Even judging it
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Where do these query changes come from?
• Given TREC Session se`ngs, we consider two sources of query change: – the previous search results that a user viewed/read/examined
– the informa"on needs • Example:
– Kurosawa à Kurosawa wife – `wife’ is not in any previous results, but in the topic descrip"on
• However, knowing informa"on needs before search is difficult to achieve
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Second take-‐home message: Previous search results influence query change
• Our observa"ons suggest that documents that have been examined by the user factor in deciding the next query change.
1−←Δ ii Dq
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Previous search results could influence query change in quite complex ways
• Merck lobbyists à Merck lobbying US policy • D1 contains several men"ons of ‘policy’, such as
– “A lobbyist who un"l 2004 worked as senior policy advisor to Canadian Prime Minister Stephen Harper was hired last month by Merck …”
• These men"ons are about Canadian policies; while the user adds US policy in q2
• Our guess is that the user might be inspired by ‘policy’, but he/she prefers a different sub-‐concept other than `Canadian policy’
• Therefore, for the added terms `US policy’, ‘US’ is the novel term here, and ‘policy’ is not since it appeared in D1. – The two terms should be treated differently
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Outline
• Introduc"on to session search • What is Query Change and Why it is important for session search?
• Where do these query changes come from? (Why the user makes those query changes?)
• How to model query changes and the dynamics in session to help retrieval?
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Markov Decision Process
• We propose to model session search as a Markov decision process (MDP)
• Two agents: the User and the Search Engine
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Se`ngs of the Session MDP
• States: Queries • Environments: Search results • Ac"ons:
– User ac"ons: • Add/remove/ unchange the query terms
– Nicely correspond to our defini"on of query change
– Search Engine ac"ons: • Increase/ decrease /remain term weights
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Search Engine Agent’s Ac"ons ∈ Di−1 action Example
qtheme
Y increase “pocono mountain” in s6
N increase “france world cup 98 reaction” in s28, france world cup 98 reaction stock market→ france world cup 98 reaction
+∆q Y decrease ‘policy’ in s37, Merck lobbyists → Merck
lobbyists US policy
N increase ‘US’ in s37, Merck lobbyists → Merck lobbyists US policy
−∆q Y decrease
‘reaction’ in s28, france world cup 98 reaction → france world cup 98
N No change
‘legislation’ in s32, bollywood legislation →bollywood law
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Modeling Query Change • A framework that is inspired by Reinforcement Learning
• Reinforcement Learning for Markov Decision Process – models a state space S and an ac"on space A according to a transi"on model T = P(si+1|si ,ai)
– a policy π(s) = a indicates that at a state s, what are the ac"ons a can be taken by the agent
– each state is associated with a reward func"on R that indicates possible posi"ve reward or nega"ve loss that a state and an ac"on may result.
• Reinforcement learning offers general solu"ons to MDP and seeks for the best policy for an agent.
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Reinforcement Learning
• In a MDP, it is believed that a future reward is not worth quite as much as a current reward and thus a discount factor γ ϵ (0,1) is applied to future rewards.
• Bellman Equa"on gives the op"mal value (expected long term reward star"ng from state s and con"nuing with policy π from then on) for an MDP:
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V*(s) = maxa
R(s,a) + γ P(s' | s,a)s '∑ V*(s')
Our Tweak
• In a MDP, it is believed that a future reward is not worth quite as much as a current reward and thus a discount factor γ ϵ (0,1) is applied to future rewards.
• In session search, a past reward is not worth quite as much as a current reward and thus a discount factor γ should be applied to past rewards – We model the MDP for session search in a reverse order
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Our Tweak & QCM
• Bellman Equa"on gives the op"mal value for an MDP:
• The reward func"on is used as the document relevance score func"on and is tweaked backwards from Bellman equa"on:
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V*(s) = maxa
R(s,a) + γ P(s' | s,a)s '∑ V*(s')
Score(qi, d) = P (qi|d) + γ P (qi|qi-1, Di-1, a)maxDi−1
P (qi-1|Di-1)a∑
Query Change retrieval Model (QCM)
Query Change retrieval Model (QCM)
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Score(qi, d) = P (qi|d) + γ P (qi|qi-1, Di-1, a)maxDi−1
P (qi-1|Di-1)a∑
Document relevant score
Discount factor
Query Transi;on model
Maximum past reward/relevance
score
Current reward/relevance score
Es"ma"ng the Transi"on Model
Score(qi, d) = log P(qi|d) +α [1−P(t | di−1* )]
t∈qtheme∑ logP(t | d)
−β P(t | di−1* )
t∈+Δqt∈di−1
*
∑ logP(t | d)+ε idf (t)t∈+Δqt∉di−1
*
∑ logP(t | d)
−δ P(t | di−1* )
t∈−Δq∑ logP(t | d)
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• According to Query Change and Search Engine Ac"ons
Current reward/ relevance score
Increase weights for theme terms
Decrease weights for removed terms
Increase weights for novel added
terms Decrease weights for old added
terms
Maximizing the Reward Func"on
• Generate a maximum rewarded document denoted as d*i-‐1, from Di-‐1 – That is the document(s) most relevant to qi-‐1
• The relevance score can be calculated as
• From several op"ons, we choose to only use the document with top
maxDi−1
P(qi−1 |Di−1)
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Scoring the En"re Session
• The overall relevance score for a session of queries is aggregated recursively :
Scoresession (qn, d) = Score(qn, d) + γScoresession (qn-1, d)= Score(qn, d) + γ[Score(qn-1, d) + γScoresession (qn-2, d)]
= γ n−i
i=1
n
∑ Score(qi, d)
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Experiments • TREC 2011-‐2012 query sets, datasets
• ClubWeb09 Category B • Systems under comparison:
– Baseline: the TREC best system’ scores (from University of Pi^sburgh) – Lemur – Nugget (our TREC 2012 Submission) – TREC Median – QCM, the proposed algorithm in this paper – QCM+De-‐Duplicate, the proposed algorithm in this paper
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Search Accuracy (TREC 2012)
• nDCG@10 (official metric used in TREC)
Approach nDCG@10 %chg MAP %chg
Lemur 0.2474 -21.54% 0.1274 -18.28%
TREC’12 median 0.2608 -17.29% 0.1440 -7.63%
Our TREC’12 submission 0.3021 −4.19% 0.1490 -4.43%
TREC’12 best 0.3221 0.00% 0.1559 0.00%
QCM 0.3353 4.10%† 0.1529 -1.92%
QCM+Dup 0.3368 4.56%† 0.1537 -1.41%
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Search Accuracy (TREC 2011)
• nDCG@10 (official metric used in TREC)
Approach nDCG@10 %chg MAP %chg
Lemur 0.3378 -23.38% 0.1118 -25.86%
TREC’11 median 0.3544 -19.62% 0.1143 -24.20%
TREC’11 best 0.4409 0.00% 0.1508 0.00%
QCM 0.4728 7.24%† 0.1713 13.59%†
QCM+Dup 0.4821 9.34%† 0.1714 13.66%†
Our TREC’12 submission 0.4836 9.68%† 0.1724 14.32%†
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Search Accuracy for Different Session Types
• TREC 2012 Sessions are classified into: – Product: Factual / Intellectual – Goal quality: Specific / Amorphous
Intellectual %chg Amorphous %chg Specific %chg Factual %chg
TREC best 0.3369 0.00% 0.3495 0.00% 0.3007 0.00% 0.3138 0.00%
Nugget 0.3305 -1.90% 0.3397 -2.80% 0.2736 -9.01% 0.2871 -8.51%
QCM 0.3870 14.87% 0.3689 5.55% 0.3091 2.79% 0.3066 -2.29%
QCM+DUP 0.3900 15.76% 0.3692 5.64% 0.3114 3.56% 0.3072 -2.10%
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-‐ BeBer handle sessions that demonstrate evolu;on and explora;on in nature than most exis;ng systems do
-‐ Because QCM treats a session as a con;nuous process by studying changes among query transi;ons and modeling the dynamics
Conclusions I
• We present a novel session search approach (QCM) by u"lizing query change and modeling the dynamic of the en"re session as a MDP
• We assume that – Query change is an important form of feedback – Query change is determined by previous search results and informa"on need
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Conclusions II • We benefit from trus"ng the user
– We believe that the most direct and valuable feedback is the next query that the user enters
• We did not do any thorough user study – More detailed analysis about user intent might be useful for us to understand web users, however, it might be overwhelming (too fine-‐grained or too much seman"cs) for a naïve search engine that essen"ally only counts words
• This research is perhaps the first to employ reinforcement learning in session search.
• Our MDP view of modeling session search can poten"ally benefit a wide range of IR tasks.
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Parameter Tuning
•
• α= 2.2, β = 1.8, γ= 0.07, and δ = 0.4.
Score(qi, d) = log P(qi|d) +α [1−P(t | di−1* )]
t∈qtheme∑ logP(t | d)
−β P(t | di−1* )
t∈+Δqt∈di−1
*
∑ logP(t | d)+ε idf (t)t∈+Δqt∉di−1
*
∑ logP(t | d)
−δ P(t | di−1* )
t∈−Δq∑ logP(t | d)
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Pa^erns of Query Changes • Specific to general
• france world cup 98 reac"on à France world cup 98 • General to specific
• pocono mountains à pocono mountains park • Concept dri�ing
• pocono mountains park à pocono mountains shopping • Slightly different expression for the same informa"on
need • glass blowing science à scien"fic glass blowing
• May even seem random – gun homicides australia àmar"n bryant port arthur massacre
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