11. Computer readable media comprising program code that when executed by a programmable processor causes execution of a method for learning ranking functions to determine the ranking of one or more content items that are responsive to a query, the computer readable media comprising: program code for generating one or more training sets comprising one or more content item-query pairs; program code for determining one or more contradicting pairs in a given training set; program code for formulating an optimization function to minimize the number of contradicting pairs in the training set using a functional iterative method that comprises applying an isotonic regression function within each query and using the output to determine regression targets for each content item-query pair in a next iteration; program code for modifying the optimization function by incorporating a grade difference between one or more content items corresponding to the query in the training set; program code for applying the optimization function to each query in the training set; program code for determining a ranking function based on an application of regression trees on the one or more queries of the training set minimized by the optimization function; and program code for storing the ranking function for application to content item-query pairs not contained in the one or more training sets.
11. Computer readable media comprising program code that when executed by a programmable processor causes execution of a method for learning ranking functions to determine the ranking of one or more content items that are responsive to a query, the computer readable media comprising: program code for generating one or more training sets comprising one or more content item-query pairs; program code for determining one or more contradicting pairs in a given training set; program code for formulating an optimization function to minimize the number of contradicting pairs in the training set using a functional iterative method that comprises applying an isotonic regression function within each query and using the output to determine regression targets for each content item-query pair in a next iteration; program code for modifying the optimization function by incorporating a grade difference between one or more content items corresponding to the query in the training set; program code for applying the optimization function to each query in the training set; program code for determining a ranking function based on an application of regression trees on the one or more queries of the training set minimized by the optimization function; and program code for storing the ranking function for application to content item-query pairs not contained in the one or more training sets. 13. The computer readable media of claim 11 , wherein program code for generating one or more training sets comprising one or more content item-query pairs further comprises program code for extracting query dependent features for the one or more content item-query pairs.