Selecting questions for a challenge-response test

Abstract

Methods, systems, and apparatus, including compute program products, for selecting a question for a challenge-response test. One of the methods includes obtaining question response pairs, wherein each question response pair includes a question about a first entity and a response to the question. Respective difficulty values for each question response pair are determined from a count of a number of times the question in the question response pair has been submitted as a search query to a search engine by users of the search engine. A request for a challenge-response test is received, wherein the request includes a context for the challenge-response test. Difficulty criteria for the challenge-response test are determined from the context. A first question response pair is selected that has a difficulty value that satisfies the difficulty criteria for the challenge-response test, the question from the first question response pair is provided in response to the request.