Identification precision of some individual prepositions even reached 100%, indicating that the rules can describe the linguistic information of PPs more accurately, especially for those PP chunks with long distance and collocations.
recall rates of rule-based method were much lower than CRF, which were also clearly reflected in the line chart, there exists significant differences between the recall rates of various prepositions, what's more, fluctuation ranges of recall rates of rule-based method were greater than CRF.