Eckhard Bick - VISL

ules appear to be slightly more complex than non-heuristic rules, the reason for this
being the fact that the heuristic level distinction is also used for non-heuristic purposes:
- for all but the mapping rules it is the only way to determine in which order rules will
be applied. Also, some of the hard, multi-context, cases are postponed to heuristic level
1, because there is a hope that other rules will resolve or restrict the ambiguity in
question in some indirect way. This double functionality of heuristic level 1 can even be
seen in the statistics in table (4), as a double peak curve in the morf1 column. The first
peak (1 context) reflects pure heuristic uses, like the removal of readings with a
tag, the other (4 contexts) is related to rule ordering and the postponement of hard cases.
While both a high remove/select ratio and a high percentage of safe contexts
(NOT and C) make a grammar more cautious (and robust), they also make the parser a
little slower. Among other things, table (5) contains the data necessary to understand the
second part of this trade-off, which is related to context type distribution. The relevant
parameter, C-percent, measures "certainty" and is computed as the ratio between the
combined number of NOT and C conditions and the number ofall contexts at a given
position. For the zero position (the target itself) the current cg-compilers do not permit
C-conditions, so here, the "safe" portion will consist of the NOT conditions alone.
(5a) Context position, polarity (±NOT) and certainty (±C)
[absolute contexts]
number of
contexts
morf syn map all
morf0 morf1 morf2 morf3 syn0 syn1 syn2 syn3
0 554 181 20 53 812 258 36 13 473 2400
NOT 0 268 92 8 2 230 48 4 - 43 695
all 0 822 1042 516 3095
C-percent 32.6 22.1 8.3 22.5
+1 250 78 2 2 97 15 8 - 220 672
+1C 310 48 1 - 28 1 - - 4 392
NOT 1 191 66 4 3 62 11 2 - 133 572
all +1 751 187 357 1636
C-percent 66.7 48.1 38.4 59.0
-1 409 103 21 12 177 29 1 - 468 1218
-1C 381 52 - 6 43 - - - 3 485
NOT -1 275 103 13 4 70 20 - - 62 547
all -1 1065 290 533 2250
C-percent 61.6 39.0 12.2 45.9
+2 42 18 - - 32 - - - 41 133
+2C 73 5 - - 24 - - - - 102
NOT 2 53 6 - - 13 1 - - 8 81
- 174 -