We implemented different variations on the basic idea of keyness: from a simple ratio of the smoothed fre-quencies to Pearson's X 2 statistic, to an interval estimate of the odds (Johnson, 2001).
we found that a more intuitive way than comparing all rules or all rule bigrams found in the treebank was to compare just the rule bigrams for one particular category -Tiger has less than 25 categories of nonterminals, so it is definitely feasible to look at these individually -and to display these visually.