-
Notifications
You must be signed in to change notification settings - Fork 36
/
Copy pathlearn-lang-diary-part-one.lyx
33353 lines (25490 loc) · 669 KB
/
learn-lang-diary-part-one.lyx
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
#LyX 2.3 created this file. For more info see http://www.lyx.org/
\lyxformat 544
\begin_document
\begin_header
\save_transient_properties true
\origin unavailable
\textclass article
\begin_preamble
\usepackage{url}
\end_preamble
\use_default_options false
\maintain_unincluded_children false
\language english
\language_package default
\inputencoding utf8
\fontencoding global
\font_roman "times" "default"
\font_sans "helvet" "default"
\font_typewriter "cmtt" "default"
\font_math "auto" "auto"
\font_default_family default
\use_non_tex_fonts false
\font_sc false
\font_osf false
\font_sf_scale 100 100
\font_tt_scale 100 100
\use_microtype false
\use_dash_ligatures false
\graphics default
\default_output_format default
\output_sync 0
\bibtex_command default
\index_command default
\paperfontsize default
\spacing single
\use_hyperref true
\pdf_bookmarks true
\pdf_bookmarksnumbered false
\pdf_bookmarksopen false
\pdf_bookmarksopenlevel 1
\pdf_breaklinks true
\pdf_pdfborder true
\pdf_colorlinks true
\pdf_backref false
\pdf_pdfusetitle true
\papersize default
\use_geometry false
\use_package amsmath 2
\use_package amssymb 2
\use_package cancel 1
\use_package esint 0
\use_package mathdots 1
\use_package mathtools 1
\use_package mhchem 0
\use_package stackrel 1
\use_package stmaryrd 1
\use_package undertilde 1
\cite_engine basic
\cite_engine_type default
\biblio_style plain
\use_bibtopic false
\use_indices false
\paperorientation portrait
\suppress_date false
\justification true
\use_refstyle 0
\use_minted 0
\index Index
\shortcut idx
\color #008000
\end_index
\secnumdepth 3
\tocdepth 3
\paragraph_separation indent
\paragraph_indentation default
\is_math_indent 0
\math_numbering_side default
\quotes_style english
\dynamic_quotes 0
\papercolumns 1
\papersides 1
\paperpagestyle default
\listings_params "basicstyle={\ttfamily},basewidth={0.5em}"
\tracking_changes false
\output_changes false
\html_math_output 0
\html_css_as_file 0
\html_be_strict false
\end_header
\begin_body
\begin_layout Title
Language Learning Diary - Part One
\end_layout
\begin_layout Date
2014-2020
\end_layout
\begin_layout Author
Linas Vepstas
\end_layout
\begin_layout Abstract
The language-learning effort involves research and software development
to implement the ideas described in ArXiv abs/1401.3372
\begin_inset CommandInset citation
LatexCommand cite
key "Goertzel2014"
literal "true"
\end_inset
.
This document contains supplementary notes and a loosely-organized semi-chronol
ogical diary of results.
\end_layout
\begin_layout Abstract
Because this document has repeatedly become overly large, it has been split
into multiple sub-documents.
Notable ones include the report on connector sets, the preliminary report
on grammatical classes, and several reports on word-pairs.
What remains here are assorted ad hoc commentary.
\end_layout
\begin_layout Abstract
Because what remains here is still too long to manage, the diary resumes
in Part Two, as of 2021.
\end_layout
\begin_layout Section*
Introduction
\end_layout
\begin_layout Standard
The language-learning effort involves research and software development
to implement the ideas described in in ArXiv abs/1401.3372
\begin_inset CommandInset citation
LatexCommand cite
key "Goertzel2014"
literal "true"
\end_inset
.
This document contains supplementary notes and a loosely-organized semi-chronol
ogical diary of results.
Its not actually chronological: in general, it is organized so that theory
precedes data analysis.
Usually.
\end_layout
\begin_layout Standard
The initial stages of this work require the extraction of word-pair probabilitie
s from raw text, and the use of these to induce a Link Grammar
\begin_inset CommandInset citation
LatexCommand cite
key "Sleator1991,Sleator1993"
literal "true"
\end_inset
.
This extends prior work on MST parsers
\begin_inset CommandInset citation
LatexCommand cite
key "Yuret1998"
literal "true"
\end_inset
, by inducing link types for word-pair relations.
\end_layout
\begin_layout Standard
Later stages further extend beyond what is possible with Link Grammar by
inducing synonymous words and phrases.
The goal here is to unify into a consistent framework various techniques
for unsupervised semantic discovery that have already been proven in narrower
contexts
\begin_inset CommandInset citation
LatexCommand cite
key "Poon2009,Lin1998,Lin2001"
literal "true"
\end_inset
.
\end_layout
\begin_layout Standard
The first section of this document is a review of various definitions of
probabilities that can be obtained from natural language text.
This is followed by a roughly chronological diary of further observaions
and results.
Many revisions are made out of chronological order.
\end_layout
\begin_layout Subsection*
Lexical Attraction, Mutual Information, Interaction Information
\end_layout
\begin_layout Standard
The goal of this section is to clarify some of the formulas used by Deniz
Yuret in his PhD thesis
\begin_inset Quotes eld
\end_inset
\emph on
Discovery of Linguistic Relations Using Lexical Attraction
\emph default
\begin_inset Quotes erd
\end_inset
, MIT 1998 (
\begin_inset CommandInset href
LatexCommand href
name "http://www2.denizyuret.com/pub/yuretphd.pdf"
target "http://www2.denizyuret.com/pub/yuretphd.pdf"
literal "false"
\end_inset
).
These formulas are vitally important, because they provide a strong tool
when working with text; this has been shown by Yuret in his thesis, as
well as by many others, as well as by my own practical experience with using
them.
\end_layout
\begin_layout Standard
Possibly the most useful formula is the one in the middle of page 40.
By the time that we get to it, the terms
\begin_inset Quotes eld
\end_inset
mutual information
\begin_inset Quotes erd
\end_inset
and
\begin_inset Quotes eld
\end_inset
lexical attraction
\begin_inset Quotes erd
\end_inset
are being used interchangeably.
This formula states the
\begin_inset Formula $MI(x,y)$
\end_inset
for two words
\begin_inset Formula $x$
\end_inset
and
\begin_inset Formula $y$
\end_inset
; yet it is manifestly not symmetric in
\begin_inset Formula $x$
\end_inset
and
\begin_inset Formula $y$
\end_inset
, since
\begin_inset Formula $x$
\end_inset
is the word on the left, and
\begin_inset Formula $y$
\end_inset
is the word on the right.
By contrast, textbook (wikipedia) definitions of MI are symmetric in their
variables.
Below I try to dis-entangle the resulting confusion a bit, and give a more
correct derivation of the formula.
The key is to observe that the formula contains an implicit pair-wise relations
hip between two words, and that there are actually three variables: two
words, and their relationship.
If this implicit relationship is made explicit, then the confusion evaporates.
It also opens the door to talking about the MI (or the interaction information
InI) of more complex relationships, not just pair-wise ones.
\end_layout
\begin_layout Standard
Being able to correctly write down the MI and the InI for complex relationships
is important for NLP: relationtionships can be labelled by types (subject,
object) and by word classes (noun, verb), and have various dependency constrain
ts between them.
Thus, we need to be able to talk both about a labelled directed graph,
and the entropy or mutual information contained in it's various sub-graphs.
\end_layout
\begin_layout Standard
In defense of Yuret, he does say, on page 22, that
\begin_inset Quotes eld
\end_inset
lexical attraction is the likelihood of a syntactic relation.
\begin_inset Quotes erd
\end_inset
However, the relation starts becoming implicit by eqn 12 on page 29.
An unexplained leap is then made from eqn 12 to the formula on page 40.
The below gets fairly pedantic; this seems unavoidable to avoid confusion.
\end_layout
\begin_layout Subsubsection*
Definitions
\end_layout
\begin_layout Standard
Let
\begin_inset Formula $P(R(w_{l},w_{r}))$
\end_inset
represent the probability (frequency) of observing two words,
\begin_inset Formula $w_{l}$
\end_inset
and
\begin_inset Formula $w_{r}$
\end_inset
in some relationship or pattern
\begin_inset Formula $R$
\end_inset
.
Typically,
\begin_inset Formula $R$
\end_inset
can be a (link-grammar) linkage of type
\begin_inset Formula $t$
\end_inset
connecting word
\begin_inset Formula $w_{l}$
\end_inset
on the left to word
\begin_inset Formula $w_{r}$
\end_inset
on the right; implicitly, both
\begin_inset Formula $w_{l}$
\end_inset
and
\begin_inset Formula $w_{r}$
\end_inset
occur in the same sentence.
The goal of this discussion is to enable relations
\begin_inset Formula $R$
\end_inset
that are more general than this; for now, though,
\begin_inset Formula $R$
\end_inset
is a word-pair occurring in a single sentence.
\end_layout
\begin_layout Standard
The simplest dependency grammar language model has only one type
\begin_inset Formula $t$
\end_inset
, the ANY type.
This is the type that Yuret uses: it makes no distinction at all between
subject, object relations (that is, all dependencies are unlabelled), and
it does not make a head-dependent distinction (all dependencies are bi-directio
nal).
Thus, in what follows, we do the same: initially, the relation
\begin_inset Formula $R(w_{l},w_{r})$
\end_inset
is simply the statement that the words
\begin_inset Formula $w_{l}$
\end_inset
and
\begin_inset Formula $w_{r}$
\end_inset
are connected by an unlabelled, un-directed edge.
For this simplest case, what
\begin_inset Formula $R(w_{l},w_{r})$
\end_inset
does is to capture that
\begin_inset Formula $w_{l}$
\end_inset
is to the left of
\begin_inset Formula $w_{r}$
\end_inset
.
\end_layout
\begin_layout Standard
In what follows, the relation
\begin_inset Formula $R=R(w_{l},w_{r})$
\end_inset
refers to a generic two-word relation, and not necessarily this simplest
one.
To regain Yuret's formula, use the simplest relation, the ordered word-pair
relation, given just above.
\end_layout
\begin_layout Standard
The quantity of interest is the (unconditional) probability
\begin_inset Formula $P(R(w_{l},w_{r}),w_{l},w_{r})$
\end_inset
of observing the two words
\begin_inset Formula $w_{l}$
\end_inset
and
\begin_inset Formula $w_{r}$
\end_inset
in a relation
\begin_inset Formula $R=R(w_{l},w_{r})$
\end_inset
.
To correctly understand and work with this quantity, some care must be
taken with the notation for several related probabilities.
First, one has
\begin_inset Formula $P(w)$
\end_inset
, the probability of observing the word
\begin_inset Formula $w$
\end_inset
in the data sample.
Next, one has
\begin_inset Formula $P(S(w_{1},w_{2}),w_{1},w_{2})$
\end_inset
, the probability that the two words occur in the same sentence.
Again,
\begin_inset Formula $S(w_{1},w_{2})$
\end_inset
denotes a relation between the two words; it differs from
\begin_inset Formula $R(w_{1},w_{2})$
\end_inset
in that the word-order does not matter.
A third kind of pair relation is the unconditional probability of observing
two words, which can be
\emph on
defined
\emph default
as
\begin_inset Formula $P(w_{1},w_{2})=P(w_{1})P(w_{2})$
\end_inset
.
In this case, instead of assuming independence of two random variables,
we define them to be so.
This is possible, because we have a notation for specifying when there
is a correlation.
That is, if there was some correlation (relation)
\begin_inset Formula $C(w_{1},w_{2})$
\end_inset
between them, then one should write this explicitly, as
\begin_inset Formula $P(C,w_{1},w_{2})=P(C(w_{1},w_{2}),w_{1},w_{2})$
\end_inset
.
The notation here allows the various needed probabilities to be defined
without ambiguity.
\end_layout
\begin_layout Standard
Thus, assumptions of independent variables are now replaced by a notational
infrastrcture.
Note, in particular, that if one uses a frequentist definition for the
probabilities (as will be done in what follows), then the probabilities
are not independent of the data sample from which they are drawn.
Thus, all probabilities here have an implicit dependence on the data sample.
This dependency is not explicitly shown.
Some care must be taken to use the same data sample throughout.
\end_layout
\begin_layout Standard
The above notation allows the definition of conditional probabilities, in
the conventional sense.
For example, one has that
\begin_inset Formula
\[
P(R,w_{l},w_{r})=P(R|w_{l},w_{r})P(w_{l},w_{r})
\]
\end_inset
or that
\begin_inset Formula
\[
P(R|w_{l},w_{r})=\frac{P(R,w_{l},w_{r})}{P(w_{l},w_{r})}
\]
\end_inset
as the conditional probability of observing the relation
\begin_inset Formula $R$
\end_inset
, given that it's component parts are observed.
From the earlier definitions, the denominator factors, and so we conclude
that the correct expression for the conditional probability is:
\begin_inset Formula
\begin{equation}
P(R|w_{l},w_{r})=\frac{P(R,w_{l},w_{r})}{P(w_{l})P(w_{r})}\label{eq:cond-pair}
\end{equation}
\end_inset
This is the probability of observing the relationship
\begin_inset Formula $R$
\end_inset
given that the individual parts of the relationship have been observed.
The relation
\begin_inset Formula $R$
\end_inset
includes all correlations between the two words: their ordering as well
as their co-occurance in a sentence.
\end_layout
\begin_layout Standard
Take care, however:
\begin_inset Formula $P(R|w_{l},w_{r})$
\end_inset
is NOT the probability of seeing
\begin_inset Formula $R$
\end_inset
, given that
\begin_inset Formula $w_{l}$
\end_inset
and
\begin_inset Formula $w_{r}$
\end_inset
occur in the same sentence.
This would instead by given by
\begin_inset Formula $P(R,w_{l},w_{r})/P(S,w_{l},w_{r})$
\end_inset
.
This is an entirely different.
\end_layout
\begin_layout Subsubsection*
Frequentism - Counting words and pairs
\end_layout
\begin_layout Standard
In order to be usable, a computable definition for the probabilities must
be given.
For this, the definition can only be frequentist.
That is, the probabilities are to be obtained from empircal data; from
counting frequencies as they occur in data samples taken from nature.
The frequency
\begin_inset Formula $P(w)$
\end_inset
of observing a word
\begin_inset Formula $w$
\end_inset
is obvious:
\begin_inset Formula
\[
P(w)=\frac{N(w)}{N(*)}
\]
\end_inset
where
\begin_inset Formula $N(w)$
\end_inset
is the count of observing word
\begin_inset Formula $w$
\end_inset
and
\begin_inset Formula $N(*)$
\end_inset
is the total number of words observed.
That is, by definition, it is the wild-card summation
\begin_inset Formula
\[
N(*)=\sum_{w}N(w)
\]
\end_inset
How to count words is not entirely obvious, so even these definitions need
care.
There are several ways in which one can count words.
One way is to simply count how many times a word occurs in the block of
sample text.
Another way is to count how many times a word occurs in parses of the sample
text.
These are not the same! For example, if a parse connects words by edges
(by dependency-grammar relations), then one can count each word once, for
each time that it occurs at the end of an edge.
In this counting, the word-count is exactly double the word-pair count.
A word is then counted multiple times, if it participates in multiple edges.
If the sample text is parsed multiple times, then additional counts can
result that way.
To maintain consistency with the definitions given in the previous section,
\begin_inset Formula $N(w)$
\end_inset
is defined to be the number of times that the word
\begin_inset Formula $w$
\end_inset
occurs in the data sample, and independent of any other relations that
\begin_inset Formula $w$
\end_inset
might be engaged in.
For now, it is assumed that the segmentation of the text sample into words
is unambiguous.
\end_layout
\begin_layout Standard
Let
\begin_inset Formula $F(S(w),w)$
\end_inset
be the number of times (frequency) of observing word
\begin_inset Formula $w$
\end_inset
in any sentence
\begin_inset Formula $S$
\end_inset
.
This can be computed as
\begin_inset Formula
\[
F(S,w)=\frac{N(w)}{NS}
\]
\end_inset
where
\begin_inset Formula $N(w)$
\end_inset
is the number of times a word
\begin_inset Formula $w$
\end_inset
was observed in a data sample, and
\begin_inset Formula $NS$
\end_inset
is the number of sentences in that same sample.
This counts with
\begin_inset Quotes eld
\end_inset
multiplicity
\begin_inset Quotes erd
\end_inset
, in that
\begin_inset Formula $w$
\end_inset
can appear in a sentence more than once.
That is,
\begin_inset Formula $F$
\end_inset
is not a probability, rather, it is an expectation value of the number
of times that a word is observed.
This can be made explicit, by writing
\end_layout
\begin_layout Standard
\begin_inset Formula
\[
F(S,w)=\frac{N(w)}{N(*)}\,\frac{N(*)}{NS}=P(w)L(S)
\]
\end_inset
with
\begin_inset Formula $L(S)=F(S,*)$
\end_inset
being the average sentence length (the expectation value of the number
of words in a sentence).
\end_layout
\begin_layout Standard
Three different word-pair relationships are interesting.
First, define the relation
\begin_inset Formula $S(w_{1},w_{2})$
\end_inset
as being the relation that both words
\begin_inset Formula $w_{1}$
\end_inset
and
\begin_inset Formula $w_{2}$
\end_inset
occur in the same sentence, but in arbitrary order.
It is symmetric:
\begin_inset Formula $S(w_{1},w_{2})=S(w_{2},w_{1})$
\end_inset
.
Define
\begin_inset Formula $A(w_{l},w_{r})$
\end_inset
as being the relation that both words
\begin_inset Formula $w_{l}$
\end_inset
and
\begin_inset Formula $w_{r}$
\end_inset
occur in the same sentence, and that
\begin_inset Formula $w_{l}$
\end_inset
is to the left of
\begin_inset Formula $w_{r}.$
\end_inset
By this definition, the counts for the two are related: one has that
\begin_inset Formula
\[
N(S,w_{1},w_{2})=N(A,w_{1},w_{2})+N(A,w_{2},w_{1})
\]
\end_inset
This is the symmetrized count.
\end_layout
\begin_layout Standard
Neither of
\begin_inset Formula $S$
\end_inset
or
\begin_inset Formula $A$
\end_inset
is yet the relation
\begin_inset Formula $R(w_{l},w_{r})$
\end_inset
mentioned above, which is defined as being the relation that both words
\begin_inset Formula $w_{l}$
\end_inset
and
\begin_inset Formula $w_{r}$
\end_inset
occur in the same sentence, that
\begin_inset Formula $w_{l}$
\end_inset
is to the left of
\begin_inset Formula $w_{r}$
\end_inset
, and, most importantly, that there is a link-grammar link (of type
\begin_inset Quotes eld
\end_inset
R
\begin_inset Quotes erd
\end_inset
) connecting the two.
Observe that although
\begin_inset Formula $A$
\end_inset
can be deduced from
\begin_inset Formula $S$
\end_inset
, there is no simple or obvious relation between
\begin_inset Formula $S$
\end_inset
and
\begin_inset Formula $R$
\end_inset
; these are essentially independent relations.
\end_layout
\begin_layout Standard
The way that the statistics are collected for
\begin_inset Formula $A$
\end_inset
and for
\begin_inset Formula $R$
\end_inset
are different.
To count the
\begin_inset Formula $A$
\end_inset
-type relations, one tokenizes a sentence into words, and then, counts every
possible word-pair in the sentence.
Effectively, one draws a clique of edges between the words, and then counts
each edge.
The statistics for
\begin_inset Formula $R$
\end_inset
are collected by parsing the sentence into a random planar tree, and then
counting the edges in the tree.
The result for this counting is NOT the same as that for type-
\begin_inset Formula $A$
\end_inset
edges.
The reason for this is demonstrated in depth, in the section
\begin_inset CommandInset ref
LatexCommand nameref
reference "sec:Edge-counting"
\end_inset
\begin_inset CommandInset ref
LatexCommand vpageref
reference "sec:Edge-counting"
\end_inset
, below.
\end_layout
\begin_layout Standard
Initially, there is only one link relation
\begin_inset Quotes eld
\end_inset
R
\begin_inset Quotes erd
\end_inset
between two words: this is the
\begin_inset Quotes eld
\end_inset
ANY
\begin_inset Quotes erd
\end_inset
link-type.
However, in general,
\begin_inset Quotes eld
\end_inset
R
\begin_inset Quotes erd
\end_inset
can be other kinds of link-types.
Note that
\begin_inset Quotes eld
\end_inset
R
\begin_inset Quotes erd
\end_inset
can also have a head-tail dependency order: either
\begin_inset Formula $w_{l}$
\end_inset
or
\begin_inset Formula $w_{r}$
\end_inset
can be the head-word of a directional link.
Thus, there are three different symmetrizations that can be obtained from
\begin_inset Quotes eld
\end_inset
R
\begin_inset Quotes erd
\end_inset
: by failing to make a left-right distinction, by failing to make a head-tail
distinction, and failing to do either.
\end_layout
\begin_layout Standard
The definition for the probability of observing a relation can be taken
to be
\begin_inset Formula
\begin{equation}
P(R,w_{l},w_{r})=\frac{N(R,w_{l},w_{r})}{N(R,*,*)}\label{eq:prob-relation}
\end{equation}
\end_inset
where
\begin_inset Formula
\[
N(R,*,*)=\sum_{w_{l},w_{r}}N(R,w_{l},w_{r})
\]
\end_inset
This can be roughly understood as being the conditional probability of observing
the relation
\begin_inset Formula $R(w_{l},w_{r})$
\end_inset
between two specific words, given that the relation
\begin_inset Formula $R$
\end_inset
between any two words was seen.
\end_layout
\begin_layout Standard
Is it possible to define the unconditional probability
\begin_inset Formula $P(R,*,*)$
\end_inset
of seeing the relationship? The path to the answer is not entirely straight-for
ward.
First consdier the probability
\begin_inset Formula $P(S,w_{1},w_{2})$
\end_inset
of seeing two words in the same sentence.
This probability is defined just as in eqn
\begin_inset CommandInset ref
LatexCommand ref
reference "eq:prob-relation"
\end_inset
; that is,
\begin_inset Formula $P(S,w_{1},w_{2})=N(S,w_{1},w_{2})/N(S,*,*)$