in other words the pitch tracker just didn't get a high enough probability of voicing for words  for  for , you know , five word
Hmm.
there are much fewer than in Switchboard. So the missing   We had a big missing data problem in Switchboard and , so the features weren't as reliable cuz they were often just not available.
Could it have to do with the  the lower frequency cut - off on the Switchboard ?