db/conf/cikm/SwanA99.html Russell C. Swan James Allan Extracting Significant Time Varying Features from Text. 38-45 CIKM1999/P038.pdf 1999 conf/cikm/99 CIKM http://doi.acm.org/10.1145/319950.319956 db/conf/cikm/cikm99.html#SwanA99