Collaborative crowdsourcing is an emerging paradigm where a set of workers, often with diverse and complementary skills, form groups and work together to complete complex tasks. While crowdsourcing has been used successfully in many applications, collaboration is essential for achieving a high quality outcome for a number of emerging applications such as text translation, citizen journalism and surveillance tasks. However, no crowdsourcing platform today enables the end-to-end deployment of collaborative tasks. We demonstrate Crowd4U, a volunteer-based system that enables the deployment of diverse crowdsourcing tasks with complex data-flows, in a declarative manner. In addition to treating workers and tasks as rich entities, Crowd4U also provides an easy-to-use form-based task UI. Crowd4U implements worker-to-task assignment algorithms that are appropriate for each kind of task. Once workers are assigned to tasks, appropriate worker collaboration schemes are enforced in order to enable effective result coordination.

SUMMARYIn this paper, we consider the XPath satisfiability problem under restricted DTDs called "duplicate free". For an XPath expression q and a DTD D, q is satisfiable under D if there exists an XML document t such that t is valid against D and that the answer of q on t is nonempty. Evaluating an unsatisfiable XPath expression is meaningless, since such an expression can always be replaced by an empty set without evaluating it. However, it is shown that the XPath satisfiability problem is intractable for a large number of XPath fragments. In this paper, we consider simple XPath fragments under two restrictions: (i) only a label can be specified as a node test and (ii) operators such as qualifier ([ ]) and path union (∪) are not allowed. We first show that, for some small XPath fragments under the above restrictions, the satisfiability problem is NP-complete under DTDs without any restriction. Then we show that there exist XPath fragments, containing the above small fragments, for which the satisfiability problem is in PTIME under duplicate-free DTDs.

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Suppose that we have a DTD and XML documents valid against the DTD, and consider writing an XPath query to the documents. Unfortunately, a user often does not understand the entire structure of the documents exactly, especially in the case where the documents are very large and/or complex, or the DTD has been updated but the user misses it. In such cases, the user tends to write an invalid XPath query. However, it is difficult for the user to correct the query by hand due to his/her lack of exact knowledge about the entire structure of the documents. In this paper, we propose an algorithm that finds, for an XPath query q, a DTD D, and a positive integer K, top-K XPath queries most syntactically close to q among the XPath queries conforming to D, so that a user select an appropriate query among the K queries. We also present some experimental studies.

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