An online data fusion system receives a query, probes a first source for an answer to the query, returns the answer from the first source, refreshes the answer while probing an additional source, and applies fusion techniques on data associated with an answer that is retrieved from the additional source. For each retrieved answer, the online data fusion system computes the probability that the answer is correct and stops retrieving data for the answer after gaining enough confidence that data retrieved from the unprocessed sources are unlikely to change the answer. The online data fusion system returns correct answers and terminates probing additional sources in an expeditious manner without sacrificing the quality of the answers.