“…Individual data sources will be more appropriate depending on the task, for example, broadcast news or conversational telephone speech. To reduce the mismatch between the interpolated model and the target domain of interest, interpolation weights may be tuned by minimizing the perplexity on some held-out data similar to the target domain (Jelinek and Mercer, 1980;Kneser and Steinbiss, 1993;Iyer et al, 1994;Bahl et al, 1995;Rosenfeld, 1996Rosenfeld, , 2000Jelinek, 1997;Clarkson and Robinson, 1997;Kneser and Peters, 1997;Seymore and Rosenfeld, 1997;Iyer and Ostendorf, 1999). These weights indicate the "usefulness" of each source for a particular task.…”