1. A method, implemented on at least one machine each of which has at least one processor, storage, and a communication platform connected to a network for predicting search results quality, the method comprising the steps of: receiving a search query from a user: obtaining a plurality of search results from a content source based on the search query, wherein the plurality of search results are ranked based on their relevance scores with respect to the search query; normalizing a distribution of the relevance scores of the plurality of search results in each position of the ranking; and computing a metric of the content source based on the normalized distribution of the relevance scores, wherein the metric indicates a relevance between the plurality of search results and the search query.