Methods, systems and apparatus, including computer programs encoded on a computer storage medium, for receiving aggregate user data, the aggregate user data corresponding to response rate of one or more answering users responding to requests, processing the aggregate user data to generate an analytical model, the analytical model describing a probability that an average answering user will respond to a request based on response rates of one or more time intervals, receiving a request, identifying a plurality of answering users, processing the analytical model to identify a sub-set of answering users of the plurality of answering users, and transmitting the request to each answering user of the sub-set of answering users.