Learning to Rank (LTR): A comprehensive introduction
In the age of the internet and vast amounts of data, the ability to find relevant information quickly and efficiently is crucial. Search engines, recommendation systems, and many other applications in the field of Information Retrieval (IR) face the challenge of identifying the most relevant items (e.g., web pages, products, documents) for a specific query or user context from a large number of elements and presenting them in an optimal order (ranking).
Learning to rank is a modern form of ranking based on machine learning. One of the most important researchers in this area is Marc Najork from Google. He was involved in most of the research work and thus shaped today’s Google search.
This article is based on all Learning to rank related documents in the SEO Research Suite database.
What is Learning to Rank?
Learning to Rank (LTR) or machine-learned ranking (MLR) is a field of machine learning that deals with the development of models and algorithms to learn this exact order. Instead of using simple rules or static weightings, LTR techniques utilize to train a scoring function. This function assigns each element a relevance score based on its features in relation to the query or context. The elements are then sorted according to this score to create the ranking.
LTR differs from classic classification or regression tasks.
While classification is about assigning an element to a specific category, and regression is about predicting a numerical value, the goal in ranking is to optimally sort the entire list of elements so that the most relevant elements appear at the top. The evaluation of the quality of a ranking is therefore done using special ranking metrics such as NDCG (Normalized Discounted Cumulative Gain), MRR (Mean Reciprocal Rank), or MAP (Mean Average Precision), which consider the position of the relevant elements in the list.

The Three Main Approaches in Learning to Rank: Pointwise, Listwise and Pairwise Ranking
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