Yandong Liu Nitya Narasimhan Venu Vasudevan Eugene Agichtein Is this urgent?: exploring time-sensitive information needs in collaborative question answering. 712-713 2009 SIGIR http://doi.acm.org/10.1145/1571941.1572091 conf/sigir/2009 db/conf/sigir/sigir2009.html#LiuNVA09