Sentence:  Three studies investigating the effects of <WSD>character density</WSD> on L2 Chinese character production found that the more strokes a character has, the more difficult it is to reproduce by hand (Ke, 1996a; L. Liu, 2008; Y. Xiao, 2002). 
Meaning of the word marked with <WSD> tags:  The number of strokes required to form a Chinese character.