1. A system comprising: an inference engine to produce a list of candidates that are potentially responsible for user-perceptible network problems, the candidates comprising network components including services and network elements; wherein a response to a service request is selected from a group of responses comprising: a non response, an incorrect response, an untimely correct response, and a timely correct response; and wherein the user-perceptible network problems include the non response, the incorrect response, and the untimely correct response.