Participatory sensing uses humans as intelligent sensors by asking them questions on mobile devices. It has been used to monitor effects of climate on plants, potholes in streets, and people's happiness in urban environments. These phenomena are difficult and expensive to measure with traditional electronic sensors. Previous participatory sensing projects have relied on the enthusiasm of the volunteers to notice and report events of interest. YouSense is a framework for centralized real-time

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... agement of the volunteers' efforts. It uses location tracking and model-based methods to target questions to participants only when they are in the optimal situation to contribute data. We have conducted two outdoor case studies to evaluate YouSense. Although it was effective at reducing participant effort wasted on unnecessary answers by 4× in short, focused studies, it was ineffective at targeting questions to valuable locations in longterm studies. Simulation results show that these outcomes are due to different participant behavior, and that centralized management is most effective at high numbers of questions.