The document presents an overview of Learning to Rank (LTR) as a method in information retrieval, detailing its history, algorithms, and practical applications in systems like Solr and Elasticsearch. It explains key concepts including the difference between traditional search techniques and LTR, the various LTR algorithms (pointwise, pairwise, listwise), and their evaluation metrics. Additionally, it discusses practical considerations for implementing LTR, such as acquiring labels and dealing with unbalanced datasets.

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Learning to Rank
Sujit Pal, Elsevier Labs
September 25-27, 2018
What it is, how it’s done, and what it can do for you

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Outline
• History
• Problem setup
• Learning to Rank Algorithms
• Practical Considerations
• LTR Case Studies (Solr, Elasticsearch, DIY)
• Wrap Up
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History
• 1992: Idea of LTR (or Machine Learned Ranking) first proposed
• 2003: Altavista (later acquired by Yahoo!) using LTR in its engine
• 2005: Microsoft invents RankNet, deploys in Bing
• 2008: In contrast, Google’s engine hand tuned, relies on up ~200 signals
• 2009: Yandex invents and deploys MatrixNet in its engine
• 2016: Google says RankBrain is #3 signal to its search engine
• 2016: Bloomberg contributes LTR plugin to Solr
• 2017: Open Source Connections contributes LTR plugin in Elasticsearch
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Problem Setup
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LTR Pipeline
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Image Credit: https://towardsdatascience.com/when-to-use-a-machine-learned-vs-score-based-search-ranker-aa8762cd9aa9
• Training: Build LTR
model using training
data (query, document,
label) triples
• Label is rank
• Inference: Use model to
predict label ŷ = h(x)
from unseen (query,
document) pairs

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Difference between search and LTR
• Search engines
• Use text based relevance – TF-IDF, BM25, etc.
• Unsupervised, backed by statistical models.
• LTR
• Can support different (application specific) notions of relevance. For example:
• Recommendations – depends on price, geolocation or user ratings.
• Question Answering – best text match might not return best answer, right set of features
may be hard to articulate explicitly.
• Supervised technique, needs labeled data to train.
• Just a re-ranker, search layer must return results to re-rank.
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Difference between ML and LTR
• ML solves a prediction problem (classification or regression) for a
single instance at a time.
• LTR solves a ranking problem for a list of items – objective is to find an
optimal ordering of items.
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Reasons to consider LTR
• Too many parameters to tune manually without overfitting to
particular query set.
• Ranking requirements not being met with traditional text based
search tools (including use of metadata fields).
• Availability of enough (implicit or explicit) good training data to train
LTR model.
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Learning to Rank Algorithms
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Traditional Ranking Models
• Vector Space Models
• Boolean – predicts if document relevant to query or not
• TF-IDF – rank documents by cosine similarity between document and query
• Probabilistic Models
• BM25 – rank documents by log odds of relevance to query
• LMIR – probability of document’s LM generating terms in query
• Importance based Models
• HITS – rank documents by hubness/authority (inlinks/outlinks).
• PageRank – rank document by probability of random surfer arriving on page
• Impact Factor – rank documents by number of citations
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Evaluation Metrics
• Mean Average Precision (MAP
@k)
• Mean Reciprocal Rank (MRR)
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• Normalized Discounted
Cumulative Gain (NDCG @k)
• Rank Correlation

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High Level Taxonomy of LTR Algorithms
• Pointwise – documents ranked by relevance of each (query,
document) pair
• Pairwise – documents ranked by considering priority between pairs of
(query, document) pairs
• Listwise – documents ranked by considering the entire relevance
ordering of all (query, Documents) tuples per query
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Pointwise Approach
• Input: (query, document) pair (q, d)
• Output: score indicating rank on result list
• Model: 𝒇(q, d) → score
• Regression problem (in case of numeric scores) or Classification
problem (in case of relevant/irrelevant, or multi-level classes like
Perfect/Excellent/Good/Fair/Bad)
• Ordinal regression: include ordinal relationship between labels.
• Examples: SLR (Staged Logistic Regression), Pranking
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Pairwise Approach
• Input: triples of (query, document pairs) (q, dA, dB)
• Output: one of [-1, 1]
• Model: 𝒇(q, dA, dB) → [-1, 1]
• Classification problem, learn binary classifier to predict [-1, 1] for a
given pair of (query, document pair) triples
• Goal is to minimize average number of inversions in ranking
• Examples: RankNet, RankSVM, LambdaMART
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Listwise Approach
• Input: (query, Documents {d1, d2, …, dN})
• Output: desired ranked list of documents 𝕯
• Model: 𝒇(q, {d1, d2, …, dN}) → 𝕯
• Classification problem, with indirect loss functions such as
RankCosine or KL Divergence, or smoothing IR measures (since not
directly differentiable) and applying Gradient Descent
• Examples: AdaRank, ListNET, RankCosine, SVMMap
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Commonly used Algorithms
• Linear Model
• Predicted rank is linear combination of input features
• RankNet
• Neural network based
• Good for binary (relevant/irrelevant) labels
• Weight matrix transforms input features into rank probabilities
• LambdaMART
• Tree (forest) based
• Good for multi-class labels
• Feature splits with thresholds
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Practical Considerations
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Acquiring labels
• Implicit
• Intrinsic features (words, phrases)
• Document metadata
• User Clicks
• Time spent on document
• Purchases (if applicable)
• Cheap to build but noisy
• Explicit
• Human expert rates relevancy of each document against query
• Cleaner but expensive to build
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Feature Selection
• Document Features
• Document Length
• URL Length
• Publication Date
• Number of outlinks
• PageRank
• Query Features
• Number of words
• PER or ORG in query
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• Query-Document Features
• TF-IDF, BM25 similarity
• Frequency of query in anchor
text
• Document contains query words
in title
• User Dependent Features
• Star ratings
• Age, gender
• Device

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Unbalanced Datasets
• If dataset is unbalanced, i.e., classes are not represented
approximately equally, then use under- or oversampling to balance.
• Consider using something like SMOTE for oversampling instead of
naïve oversampling by duplication.
• Make sure no data leakage in case of oversampling.
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LTR used as re-ranker
• LTR models are usually more
computationally expensive than
search engines.
• Search engine used to pull out
matched documents
• Top-N of these documents are fed into
the LTR model and top-n of those are
replaced with the output of the
model, for N >> n (typically 50-100x).
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Index
Query
Matched
(10k)
Scored
(10k)
Top 1000
retrieved
Re-ranked
Top 10
Ranking
Model
Image Credit: https://lucidworks.com/2016/08/17/learning-to-rank-solr/

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LTR Algorithm Implementations
• RankLib (Java) – from Lemur Project (UMass, CMU), provides
Coordinate Ascent, Random Forest (pointwise), MART, RankNet,
RankBoost (pairwise), LambdaMART (pair/listwise), AdaRank and
ListNet (listwise)
• SVMRank (C++) – from Cornell, provides SVMRank (pairwise)
• XGBoost (Python/C++) – LambdaRank (pairwise)
• PyLTR (Python) – LambdaMART (pairwise)
• Michael Alcorn (Python) – RankNet and LambdaMART (pairwise)
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LETOR Data Format
2 qid:1 1:3 2:3 3:0 4:0 5:3 6:1 7:1 8:0 9:0 10:1 11:156... # 11
2 qid:1 1:3 2:0 3:3 4:0 5:3 6:1 7:0 8:1 9:0 10:1 11:406... # 23
0 qid:1 1:3 2:0 3:2 4:0 5:3 6:1 7:0 8:0.666667 9:0 10:1 ... # 44
2 qid:1 1:3 2:0 3:3 4:0 5:3 6:1 7:0 8:1 9:0 10:1 11:287 ... # 57
1 qid:1 1:3 2:0 3:3 4:0 5:3 6:1 7:0 8:1 9:0 10:1 11:2009 ... # 89
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label
Query ID
Features: query, document, query/document, other
(sparse or dense format)
Comments (ex: docID)

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Case Studies
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Preprocessing Data
• We use The Movie Database (TMDB) from Kaggle.
• 45k movies, 20 genres, 31k unique keywords
• We extract following fields: (docID, title, description, popularity,
release date, running time, rating (0-10), keywords, genres)
• Categorical labels 1-5 created from rating
• Objective is to build LTR model that learns the ordering implied by
rating and re-rank top 10 results using this model
• Features chosen: (query-title and query-description similarity using
TF-IDF and BM25, document recency, original score, and boolean 0/1
for each genre)
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LTR with Solr
• Prepare Solr for LTR (add snippet to solrconfig.xml) and start with
solr.ltr.enabled=True
• Load data
• Define LTR features to be used to Solr
• Define dummy linear model to use Solr to extract features (rq) for some
queries to LETOR format
• Train RankLib LambdaMART model using extracted features
• Upload trained model definition to Solr
• Run Solr re-rank query (rq) using trained LTR model
• See notebooks – 02-solr/01 .. 04
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LTR with Elasticsearch
• Install LTR plugin and load data
• Initialize feature store
• Define features – load feature templates into Elasticsearch
• Extract features (sltr) to LETOR format
• Train RankLib model (also supported natively XGBoost, SVMRank).
• Upload trained LTR model to Elasticsearch
• Run re-rank query (rescore) using trained LTR model
• See notebooks – 03-elasticsearch/01 .. 04
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DIY LTR – Index Agnostic
• Run queries, generate features from results to LETOR format
• Train RankLib (or other third party LTR) model
• Run re-rank query on trained model
• Merge output of re-rank with actual results from index
• See notebooks – 04-ranklib/02..04
• Pros: index agnostic; more freedom to add novel features
• Cons: less support from index
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Wrapping Up
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Resources
• Book – Learning to Rank for Information Retrieval, by Tie-Yan Liu.
• Paper – From RankNet to LambdaRank to LambdaMART: An
Overview, by Christopher J. C. Burges
• Tutorials
• Solr - https://github.com/airalcorn2/Solr-LTR
• Elasticsearch – Learning to Rank 101 by Pere Urbon-Bayes, ES-LTR Demo by
Doug Turnbull.
• Product Centric LTR Documentation
• Solr Learning To Rank Docs
• Elasticsearch Learning to Rank Docs
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Thank you!
• Contact: sujit.pal@elsevier.com
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