Automatic Selection of Context Configurations for Improved Class-Specific Word Representations

08/19/2016 ∙ by Ivan Vulić, et al. ∙ Technion University of Cambridge Hebrew University of Jerusalem University of Washington 0

This paper is concerned with identifying contexts useful for training word representation models for different word classes such as adjectives (A), verbs (V), and nouns (N). We introduce a simple yet effective framework for an automatic selection of class-specific context configurations. We construct a context configuration space based on universal dependency relations between words, and efficiently search this space with an adapted beam search algorithm. In word similarity tasks for each word class, we show that our framework is both effective and efficient. Particularly, it improves the Spearman's rho correlation with human scores on SimLex-999 over the best previously proposed class-specific contexts by 6 (A), 6 (V) and 5 (N) rho points. With our selected context configurations, we train on only 14 all dependency-based contexts, resulting in a reduced training time. Our results generalise: we show that the configurations our algorithm learns for one English training setup outperform previously proposed context types in another training setup for English. Moreover, basing the configuration space on universal dependencies, it is possible to transfer the learned configurations to German and Italian. We also demonstrate improved per-class results over other context types in these two languages.

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1 Introduction

Dense real-valued word representations (embeddings) have become ubiquitous in NLP, serving as invaluable features in a broad range of tasks Turian et al. (2010); Collobert et al. (2011); Chen and Manning (2014). The omnipresent word2vec skip-gram model with negative sampling (SGNS) Mikolov et al. (2013) is still considered a robust and effective choice for a word representation model, due to its simplicity, fast training, as well as its solid performance across semantic tasks Baroni et al. (2014); Levy et al. (2015). The original SGNS implementation learns word representations from local bag-of-words contexts (BOW). However, the underlying model is equally applicable with other context types Levy and Goldberg (2014a).

Recent work suggests that “not all contexts are created equal”. For example, reaching beyond standard BOW contexts towards contexts based on dependency parses Bansal et al. (2014); Melamud et al. (2016) or symmetric patterns Schwartz et al. (2015, 2016) yields significant improvements in learning representations for particular word classes such as adjectives (A) and verbs (V). Moreover, Schwartz:2016naacl demonstrated that a subset of dependency-based contexts which covers only coordination structures is particularly effective for SGNS training, both in terms of the quality of the induced representations and in the reduced training time of the model. Interestingly, they also demonstrated that despite the success with adjectives and verbs, BOW contexts are still the optimal choice when learning representations for nouns (N).

In this work, we propose a simple yet effective framework for selecting context configurations, which yields improved representations for verbs, adjectives, and nouns. We start with a definition of our context configuration space (Sect. 3.1). Our basic definition of a context refers to a single typed (or labeled) dependency link between words (e.g., the amod link or the dobj link). Our configuration space then naturally consists of all possible subsets of the set of labeled dependency links between words. We employ the universal dependencies (UD) scheme to make our framework applicable across languages. We then describe (Sect. 3.2) our adapted beam search algorithm that aims to select an optimal context configuration for a given word class.

We show that SGNS requires different context configurations to produce improved results for each word class. For instance, our algorithm detects that the combination of amod and conj contexts is effective for adjective representation. Moreover, some contexts that boost representation learning for one word class (e.g., amod contexts for adjectives) may be uninformative when learning representations for another class (e.g., amod for verbs). By removing such dispensable contexts, we are able both to speed up the SGNS training and to improve representation quality.

We first experiment with the task of predicting similarity scores for the A/V/N portions of the benchmarking SimLex-999 evaluation set, running our algorithm in a standard SGNS experimental setup Levy et al. (2015). When training SGNS with our learned context configurations it outperforms SGNS trained with the best previously proposed context type for each word class: the improvements in Spearman’s rank correlations are 6 (A), 6 (V), and 5 (N) points. We also show that by building context configurations we obtain improvements on the entire SimLex-999 (4 points over the best baseline). Interestingly, this context configuration is not the optimal configuration for any word class.

We then demonstrate that our approach is robust by showing that transferring the optimal configurations learned in the above setup to three other setups yields improved performance. First, the above context configurations, learned with the SGNS training on the English Wikipedia corpus, have an even stronger impact on SimLex999 performance when SGNS is trained on a larger corpus. Second, the transferred configurations also result in competitive performance on the task of solving class-specific TOEFL questions. Finally, we transfer the learned context configurations across languages: these configurations improve the SGNS performance when trained with German or Italian corpora and evaluated on class-specific subsets of the multilingual SimLex-999 Leviant and Reichart (2015), without any language-specific tuning.

2 Related Work

Word representation models typically train on (word, context) pairs. Traditionally, most models use bag-of-words (BOW) contexts, which represent a word using its neighbouring words, irrespective of the syntactic or semantic relations between them (Collobert et al., 2011; Mikolov et al., 2013; Mnih and Kavukcuoglu, 2013; Pennington et al., 2014, inter alia). Several alternative context types have been proposed, motivated by the limitations of BOW contexts, most notably their focus on topical rather than functional similarity (e.g., coffee:cup vs. coffee:tea). These include dependency contexts Padó and Lapata (2007); Levy and Goldberg (2014a), pattern contexts Baroni et al. (2010); Schwartz et al. (2015)

and substitute vectors

Yatbaz et al. (2012); Melamud et al. (2015).

Several recent studies examined the effect of context types on word representation learning. Melamud:2016naacl compared three context types on a set of intrinsic and extrinsic evaluation setups: BOW, dependency links, and substitute vectors. They show that the optimal type largely depends on the task at hand, with dependency-based contexts displaying strong performance on semantic similarity tasks. Vulic:2016acluniversal extended the comparison to more languages, reaching similar conclusions. Schwartz:2016naacl, showed that symmetric patterns are useful as contexts for V and A similarity, while BOW still works best for nouns. They also indicated that coordination structures, a particular dependency link, are more useful for verbs and adjectives than the entire set of dependencies. In this work, we generalise their approach: our algorithm systematically and efficiently searches the space of dependency-based context configurations, yielding class-specific representations with substantial gains for all three word classes.

Previous attempts on specialising word representations for a particular relation (e.g., similarity vs relatedness, antonyms) operate in one of two frameworks: (1) modifying the prior or the regularisation of the original training procedure Yu and Dredze (2014); Wieting et al. (2015); Liu et al. (2015); Kiela et al. (2015); Ling et al. (2015b); (2) post-processing procedures which use lexical knowledge to refine previously trained word vectors Faruqui et al. (2015); Wieting et al. (2015); Mrkšić et al. (2017). Our work suggests that the induced representations can be specialised by directly training the word representation model with carefully selected contexts.

3 Context Selection: Methodology

Figure 1: Extracting dependency-based contexts. Top: An example English sentence from Levy and Goldberg (2014a), now UD-parsed. Middle: the same sentence in Italian, UD-parsed. Note the similarity between the two parses which suggests that our context selection framework may be extended to other languages. Bottom: prepositional arc collapsing. The uninformative short-range case arc is removed, while a “pseudo-arc” specifying the exact link (prep:with) between discovers and telescope is added.

The goal of our work is to develop a methodology for the identification of optimal context configurations for word representation model training. We hope to get improved word representations and, at the same time, cut down the training time of the word representation model. Fundamentally, we are not trying to design a new word representation model, but rather to find valuable configurations for existing algorithms.

The motivation to search for such training context configurations lies in the intuition that the distributional hypothesis Harris (1954) should not necessarily be made with respect to BOW contexts. Instead, it may be restated as a series of statements according to particular word relations. For example, the hypothesis can be restated as: “two adjectives are similar if they modify similar nouns”, which is captured by the amod typed dependency relation. This could also be reversed to reflect noun similarity by saying that “two nouns are similar if they are modified by similar adjectives”. In another example, “two verbs are similar if they are used as predicates of similar nominal subjects” (the nsubj and nsubjpass dependency relations).

First, we have to define an expressive context configuration space that contains potential training configurations and is effectively decomposed so that useful configurations may be sought algorithmically. We can then continue by designing a search algorithm over the configuration space.

3.1 Context Configuration Space

We focus on the configuration space based on dependency-based contexts (DEPS) Padó and Lapata (2007); Utt and Padó (2014). We choose this space due to multiple reasons. First, dependency structures are known to be very useful in capturing functional relations between words, even if these relations are long distance. Second, they have been proven useful in learning word embeddings Levy and Goldberg (2014a); Melamud et al. (2016). Finally, owing to the recent development of the Universal Dependencies (UD) annotation scheme McDonald et al. (2013); Nivre et al. (2016)111http://universaldependencies.org/ (V1.4 used) it is possible to reason over dependency structures in a multilingual manner (e.g., Fig. 1). Consequently, a search algorithm in such DEPS-based configuration space can be developed for multiple languages based on the same design principles. Indeed, in this work we show that the optimal configurations for English translate to improved representations in two additional languages, German and Italian.

And so, given a (UD-)parsed training corpus, for each target word with modifiers and a head , the word is paired with context elements , where is the type of the dependency relation between the head and the modifier (e.g., amod), and denotes an inverse relation. To simplify the presentation, we adopt the assumption that all training data for the word representation model are in the form of such pairs Levy and Goldberg (2014a, c), where is the current target word, and is its observed context (e.g., BOW, positional, dependency-based). A naive version of DEPS extracts contexts from the parsed corpus without any post-processing. Given the example from Fig. 1, the DEPS contexts of discovers are: scientist_nsubj, stars_dobj, telescope_nmod.

DEPS not only emphasises functional similarity, but also provides a natural implicit grouping of related contexts. For instance, all pairs with the shared relation and are taken as an -based context bag, e.g., the pairs (scientist, Australian_amod), (Australian, scientist_) from Fig. 1 are inserted into the amod context bag, while (discovers, stars_dobj), (stars, discovers_) are labelled with dobj.

Assume that we have obtained distinct dependency relations after parsing and post-processing the corpus. The -th individual context bag, , labelled , is a bag (or a multiset) of pairs where has one of the following forms: or , where is some vocabulary word. A context configuration is then simply a set of individual context bags, e.g., , also labelled as : . We call a configuration consisting of individual context bags a -set configuration (e.g., in this example, is a -set configuration).222A note on the nomenclature and notation: Each context configuration may be seen as a set of context bags, as it does not allow for repetition of its constituent context bags. For simplicity and clarity of presentation, we use dependency relation types (e.g., = amod, = acl) as labels for context bags. The reader has to be aware that a configuration is not by any means a set of relation types/names, but is in fact a multiset of all pairs belonging to the corresponding context bags labelled with , , .

Figure 2: An illustration of Alg. 1. The search space is presented as a DAG with direct links between origin configurations (e.g., ) and all its children configurations obtained by removing exactly one individual bag from the origin (e.g., , ). After automatically constructing the initial pool (line 1), the entry point of the algorithm is the configuration (line 2). Thicker blue circles denote visited configurations, while the gray circle denotes the best configuration found.
Input : Set of individual context bags:
1 build: pool of those candidate individual context bags for which , where is a fitness function.
2 build: -set configuration ;
3 initialize: (1) set of candidate configurations ; (2) current level ; (3) best configuration ;
4 search:
5 repeat
6        ;
7        ;
8        foreach  do
9               foreach  do
10                      build new -set context configuration ;
11                      if  then
12                             ;
13                            
14                     
15              
16        ;
17        ;
18       
19until  or ;
Output : Best configuration
Algorithm 1 Best Configuration Search

Although a brute-force exhaustive search over all possible configurations is possible in theory and for small pools (e.g., for adjectives, see Tab. 2), it becomes challenging or practically infeasible for large pools and large training data. For instance, based on the pool from Tab. 2, the search for the optimal configuration would involve trying out

configurations for nouns (i.e., training 1023 different word representation models). Therefore, to reduce the number of visited configurations, we present a simple heuristic search algorithm inspired by beam search

Pearl (1984).

3.2 Class-Specific Configuration Search

Alg. 1 provides a high-level overview of the algorithm. An example of its flow is given in Fig. 2. Starting from , the set of all possible individual context bags, the algorithm automatically detects the subset , , of candidate individual bags that are used as the initial pool (line 1 of Alg. 1). The selection is based on some fitness (goal) function . In our setup, is Spearman’s correlation with human judgment scores obtained on the development set after training the word representation model with the configuration . The selection step relies on a simple threshold: we use a threshold of without any fine-tuning in all experiments with all word classes.

We find this step to facilitate efficiency at a minor cost for accuracy. For example, since amod denotes an adjectival modifier of a noun, an efficient search procedure may safely remove this bag from the pool of candidate bags for verbs.

The search algorithm then starts from the full -set configuration (line 3) and tests -set configurations where exactly one individual bag is removed to generate each such configuration (line 10). It then retains only the set of configurations that score higher than the origin -set configuration (lines 11-12, see Fig. 2). Using this principle, it continues searching only over lower-level -set configurations that further improve performance over their -set origin configuration. It stops if it reaches the lowest level or if it cannot improve the goal function any more (line 15). The best scoring configuration is returned (n.b., not guaranteed to be the global optimum).

In our experiments with this heuristic, the search for the optimal configuration for verbs is performed only over 13 -set configurations plus 26 other configurations (39 out of 133 possible configurations).333The total is 133 as we have to include 6 additional -set configurations that have to be tested (line 1 of Alg. 1) but are not included in the initial pool for verbs (line 2). For nouns, the advantage of the heuristic is even more dramatic: only 104 out of 1026 possible configurations were considered during the search.444We also experimented with a less conservative variant which does not stop when lower-level configurations do not improve ; it instead follows the path of the best-scoring lower-level configuration even if its score is lower than that of its origin. As we do not observe any significant improvement with this variant, we opt for the faster and simpler one.

4 Experimental Setup

4.1 Implementation Details

Word Representation Model

We experiment with SGNS Mikolov et al. (2013), the standard and very robust choice in vector space modeling Levy et al. (2015). In all experiments we use word2vecf, a reimplementation of word2vec able to learn from arbitrary pairs.555https://bitbucket.org/yoavgo/word2vecf For details concerning the implementation, we refer the reader to Goldberg and Levy (2014); Levy and Goldberg (2014a).

The SGNS preprocessing scheme was replicated from Levy and Goldberg (2014a); Levy et al. (2015). After lowercasing, all words and contexts that appeared less than 100 times were filtered. When considering all dependency types, the vocabulary spans approximately 185K word types.666

SGNS for all models was trained using stochastic gradient descent and standard settings:

negative samples, global learning rate: , subsampling rate: , epochs. Further, all representations were trained with (very similar trends are observed with ).

The same setup was used in prior work Schwartz et al. (2016); Vulić and Korhonen (2016). Keeping the representation model fixed across experiments and varying only the context type allows us to attribute any differences in results to a sole factor: the context type. We plan to experiment with other representation models in future work.

Universal Dependencies as Labels

The adopted UD scheme leans on the universal Stanford dependencies de Marneffe et al. (2014) complemented with the universal POS tagset Petrov et al. (2012). It is straightforward to “translate” previous annotation schemes to UD de Marneffe et al. (2014). Providing a consistently annotated inventory of categories for similar syntactic constructions across languages, the UD scheme facilitates representation learning in languages other than English, as shown in Vulić and Korhonen (2016); Vulić (2017).

Individual Context Bags

Standard post-parsing steps are performed in order to obtain an initial list of individual context bags for our algorithm: (1) Prepositional arcs are collapsed (Levy and Goldberg (2014a); Vulić and Korhonen (2016), see Fig. 1). Following this procedure, all pairs where the relation has the form prep:X (where X is a preposition) are subsumed to a context bag labelled prep; (2) Similar labels are merged into a single label (e.g., direct (dobj) and indirect objects (iobj) are merged into obj); (3) Pairs with infrequent and uninformative labels are removed (e.g., punct, goeswith, cc).

Coordination-based contexts are extracted as in prior work Schwartz et al. (2016), distinguishing between left and right contexts extracted from the conj relation; the label for this bag is conjlr. We also utilise the variant that does not make the distinction, labeled conjll. If both are used, the label is simply conj=conjlr+conjll.777Given the coordination structure boys and girls, conjlr training pairs are (boys, girls_conj), (girls, , while conjll pairs are (boys, girls_conj), (girls, .

Consequently, the individual context bags we use in all experiments are: subj, obj, comp, nummod, appos, nmod, acl, amod, prep, adv, compound, conjlr, conjll.

4.2 Training and Evaluation

We run the algorithm for context configuration selection only once, with the SGNS training setup described below. Our main evaluation setup is presented below, but the learned configurations are tested in additional setups, detailed in Sect. 5.

Training Data

Our training corpus is the cleaned and tokenised English Polyglot Wikipedia data Al-Rfou et al. (2013),888https://sites.google.com/site/rmyeid/projects/polyglot consisting of approximately 75M sentences and 1.7B word tokens. The Wikipedia data were POS-tagged with universal POS (UPOS) tags Petrov et al. (2012) using the state-of-the art TurboTagger Martins et al. (2013).999http://www.cs.cmu.edu/~ark/TurboParser/ The parser was trained using default settings (SVM MIRA with 20 iterations, no further parameter tuning) on the train+dev portion of the UD treebank annotated with UPOS tags. The data were then parsed with UD using the graph-based Mate parser v3.61 Bohnet (2010)101010https://code.google.com/archive/p/mate-tools/ with standard settings on train+dev of the UD treebank.

Evaluation

We experiment with the verb pair (222 pairs), adjective pair (111 pairs), and noun pair (666 pairs) portions of SimLex-999. We report Spearman’s

correlation between the ranks derived from the scores of the evaluated models and the human scores. Our evaluation setup is borrowed from Levy:2015tacl: we perform 2-fold cross-validation, where the context configurations are optimised on a development set, separate from the unseen test data. Unless stated otherwise, the reported scores are always the averages of the 2 runs, computed in the standard fashion by applying the cosine similarity to the vectors of words participating in a pair.

4.3 Baselines

Baseline Context Types

We compare the context configurations found by Alg. 1 against baseline contexts from prior work:
- BOW: Standard bag-of-words contexts.
- POSIT: Positional contexts Schütze (1993); Levy and Goldberg (2014b); Ling et al. (2015a), which enrich BOW with information on the sequential position of each context word. Given the example from Fig. 1, POSIT with the window size extracts the following contexts for discovers: Australian_-2, scientist_-1, stars_+2, with_+1.
- DEPS-All: All dependency links without any context selection, extracted from dependency-parsed data with prepositional arc collapsing.
- COORD: Coordination-based contexts are used as fast lightweight contexts for improved representations of adjectives and verbs Schwartz et al. (2016). This is in fact the conjlr context bag, a subset of DEPS-All.
- SP: Contexts based on symmetric patterns (SPs, Davidov and Rappoport (2006); Schwartz et al. (2015)). For example, if the word X and the word Y appear in the lexico-syntactic symmetric pattern “X or Y” in the SGNS training corpus, then Y is an SP context instance for X, and vice versa.

The development set was used to tune the window size for BOW and POSIT (to 2) and the parameters of the SP extraction algorithm.111111The SP extraction algorithm is available online:
homes.cs.washington.edu/roysch/software/dr06/dr06.html

Baseline Greedy Search Algorithm

We also compare our search algorithm to its greedy variant: at each iteration of lines 8-12 in Alg. 1, now keeps only the best configuration of size that perform better than the initial configuration of size , instead of all such configurations.

Context Group Adj Verb Noun
conjlr (A+N+V) 0.415 0.281 0.401
obj (N+V) -0.028 0.309 0.390
prep (N+V) 0.188 0.344 0.387
amod (A+N) 0.479 0.058 0.398
compound (N) -0.124 -0.019 0.416
adv (V) 0.197 0.342 0.104
nummod (-) -0.142 -0.065 0.029
Table 1: 2-fold cross-validation results for an illustrative selection of individual context bags. Results are presented for the noun, verb and adjective subsets of SimLex-999. Values in parentheses denote the class-specific initial pools to which each context is selected based on its score (line 1 of Alg. 1).
Adjectives Verbs Nouns
amod, conjlr, conjll prep, acl, obj, comp, adv, conjlr, conjll amod, prep, compound, subj, obj, appos, acl, nmod, conjlr, conjll
Table 2: Automatically constructed initial pools of candidate bags for each word class (Sect. 3.2).
Baselines (Verbs) BOW (win=2) 0.336 POSIT (win=2) 0.345 COORD (conjlr) 0.283 SP 0.349 DEPS-All 0.344 Configurations: Verbs POOL-ALL 0.379 prep+acl+obj+adv+conj 0.393 prep+acl+obj+comp+conj 0.344 prep+obj+comp+adv+conj 0.391 prep+acl+adv+conj (BEST) 0.409 prep+acl+obj+adv 0.392 prep+acl+adv 0.407 prep+acl+conj 0.390 acl+obj+adv+conj 0.345 acl+obj+adv 0.385 Baselines (Nouns) BOW (win=2) 0.435 POSIT (win=2) 0.437 COORD (conjlr) 0.392 SP 0.372 DEPS-All 0.441 Configurations: Nouns POOL-ALL 0.469 amod+subj+obj+appos+compound+nmod+conj 0.478 amod+subj+obj+appos+compound+conj 0.487 amod+subj+obj+appos+compound+conjlr 0.476 amod+subj+obj+compound+conj (BEST) 0.491 amod+subj+obj+appos+conj 0.470 subj+obj+compound+conj 0.479 amod+subj+compound+conj 0.481 amod+subj+obj+compound 0.478 amod+obj+compound+conj 0.481
Table 3: Results on the SimLex-999 test data over (a) verbs and (b) nouns subsets. Only a selection of context configurations optimised for verb and noun similarity are shown. POOL-ALL denotes a configuration where all individual context bags from the verbs/nouns-oriented pools (see Table 2) are used. BEST denotes the best performing configuration found by Alg. 1. Other configurations visited by Alg. 1 that score higher than the best scoring baseline context type for each word class are in gray. Scores obtained using a greedy search algorithm instead of Alg. 1 are in italic, marked with a cross ().

5 Results and Discussion

5.1 Main Evaluation Setup

Not All Context Bags are Created Equal

First, we test the performance of individual context bags across SimLex-999 adjective, verb, and noun subsets. Besides providing insight on the intuition behind context selection, these findings are important for the automatic selection of class-specific pools (line 1 of Alg. 1). The results are shown in Tab. 2.

The experiment supports our intuition (see Sect. 3.2): some context bags are definitely not useful for some classes and may be safely removed when performing the class-specific SGNS training. For instance, the amod bag is indeed important for adjective and noun similarity, and at the same time it does not encode any useful information regarding verb similarity. compound is, as expected, useful only for nouns. Tab. 2 also suggests that some context bags (e.g., nummod) do not encode any informative contextual evidence regarding similarity, therefore they can be discarded. The initial results with individual context bags help to reduce the pool of candidate bags (line 1 in Alg. 1), see Tab. 2.

Baselines (Adjectives)
BOW (win=2) 0.489
POSIT (win=2) 0.460
COORD (conjlr) 0.407
SP 0.395
DEPS-All 0.360
Configurations: Adjectives
POOL-ALL: amod+conj (BEST) 0.546
amod+conjlr 0.527
amod+conjll 0.531
conj 0.470
Table 4: Results on the SimLex-999 adjectives subset with adjective-specific configurations.

Searching for Improved Configurations

Next, we test if we can improve class-specific representations by selecting class-specific configurations. Results are summarised in Tables 3 and 4. Indeed, class-specific configurations yield better representations, as is evident from the scores: the improvements with the best class-specific configurations found by Alg. 1 are approximately 6 points for adjectives, 6 points for verbs, and 5 points for nouns over the best baseline for each class.

The improvements are visible even with configurations that simply pool all candidate individual bags (POOL-ALL), without running Alg. 1 beyond line 1. However, further careful context selection, i.e., traversing the configuration space using Alg. 1 leads to additional improvements for V and N (gains of 3 and 2.2 points). Very similar improved scores are achieved with a variety of configurations (see Tab. 3), especially in the neighbourhood of the best configuration found by Alg. 1. This indicates that the method is quite robust: even sub-optimal121212The term optimal here and later in the text refers to the best configuration returned by our algorithm. solutions result in improved class-specific representations. Furthermore, our algorithm is able to find better configurations for verbs and nouns compared to its greedy variant. Finally, our algorithm generalises well: the best scoring configuration on the dev set is always the best one on the test set.

Training: Fast and/or Accurate?

Carefully selected configurations are also likely to reduce SGNS training times. Indeed, the configuration-based model trains on only 14% (A), 26.2% (V), and 33.6% (N) of all dependency-based contexts. The training times and statistics for each context type are displayed in Tab. 5. All models were trained using parallel training on 10 Intel(R) Xeon(R) E5-2667 2.90GHz processors. The results indicate that class-specific configurations are not as lightweight and fast as SP or COORD contexts Schwartz et al. (2016). However, they also suggest that such configurations provide a good balance between accuracy and speed: they reach peak performances for each class, outscoring all baseline context types (including SP and COORD), while training is still much faster than with “heavyweight” context types such as BOW, POSIT or DEPS-All.

Now that we verified the decrease in training time our algorithm provides for the final training, it makes sense to ask whether the configurations it finds are valuable in other setups. This will make the fast training of practical importance.

Context Type Training Time # Pairs
BOW (win=2) 179mins 27s 5.974G
POSIT (win=2) 190mins 12s 5.974G
COORD (conjlr) 4mins 11s 129.69M
SP 1mins 29s 46.37M
DEPS-All 103mins 35s 3.165G
BEST-ADJ 14mins 5s 447.4M
BEST-VERBS 29mins 48s 828.55M
BEST-NOUNS 41mins 14s 1.063G
Table 5: Training time (wall-clock time reported) in minutes for SGNS () with different context types. BEST-* denotes the best scoring configuration for each class found by Alg. 1. #Pairs shows a total number of pairs used in SGNS training for each context type.

5.2 Generalisation: Configuration Transfer

Another Training Setup

We first test whether the context configurations learned in Sect. 5.1 are useful when SGNS is trained in another English setup Schwartz et al. (2016), with more training data and other annotation and parser choices, while evaluation is still performed on SimLex-999.

Context Type Adj Verbs Nouns All
BOW (win=2) 0.604 0.307 0.501 0.464
POSIT (win=2) 0.585 0.400 0.471 0.469
COORD (conjlr) 0.629 0.413 0.428 0.430
SP 0.649 0.458 0.414 0.444
DEPS-All 0.574 0.389 0.492 0.464
BEST-ADJ 0.671 0.348 0.504 0.449
BEST-VERBS 0.392 0.455 0.478 0.448
BEST-NOUNS 0.581 0.327 0.535 0.489
BEST-ALL 0.616 0.402 0.519 0.506
Table 6: Results on the A/V/N SimLex-999 subsets, and on the entire set (All) in the setup from Schwartz:2016naacl. . BEST-* are again the best class-specific configs returned by Alg. 1.

In this setup the training corpus is the 8B words corpus generated by the word2vec script.131313code.google.com/p/word2vec/source/browse/trunk/ A preprocessing step now merges common word pairs and triplets to expression tokens (e.g., Bilbo_Baggins). The corpus is parsed with labelled Stanford dependencies de Marneffe and Manning (2008) using the Stanford POS Tagger Toutanova et al. (2003) and the stack version of the MALT parser Goldberg and Nivre (2012). SGNS preprocessing and parameters are also replicated; we now train -dim embeddings as in prior work.141414The “translation” from labelled Stanford dependencies into UD is performed using the mapping from Marneffe:2014lrec, e.g., nn is mapped into compound, and rcmod, partmod, infmod are all mapped into one bag: acl.

Results are presented in Tab. 6. The imported class-specific configurations, computed using a much smaller corpus (Sect. 5.1), again outperform competitive baseline context types for adjectives and nouns. The BEST-VERBS configuration is outscored by SP, but the margin is negligible. We also evaluate another configuration found using Alg. 1 in Sect. 5.1, which targets the overall improved performance without any finer-grained division to classes (BEST-ALL). This configuration (amod+subj+obj+compound+prep+adv+conj) outperforms all baseline models on the entire benchmark. Interestingly, the non-specific BEST-ALL configuration falls short of A/V/N-specific configurations for each class. This unambiguously implies that the “trade-off” configuration targeting all three classes at the same time differs from specialised class-specific configurations.

Context Type Adj-Q Verb-Q Noun-Q
BOW (win=2) 31/41 14/19 16/19
POSIT (win=2) 32/41 13/19 15/19
COORD (conjlr) 26/41 11/19   8/19
SP 26/41 11/19 12/19
DEPS-All 31/41 14/19 16/19
BEST-ADJ 32/41 12/19 15/19
BEST-VERBS 24/41 15/19 16/19
BEST-NOUNS 30/41 14/19 17/19
Table 7: Results on the A/V/N TOEFL question subsets. The reported scores are in the following form: correct_answers/overall_questions. Adj-Q refers to the subset of TOEFL questions targeting adjectives; similar for Verb-Q and Noun-Q. BEST-* refer to the best class-specific configurations from Tab. 3 and Tab. 4.

Experiments on Other Languages

We next test whether the optimal context configurations computed in Sect. 5.1 with English training data are also useful for other languages. For this, we train SGNS models on the Italian (IT) and German (DE) Polyglot Wikipedia corpora with those configurations, and evaluate on the IT and DE multilingual SimLex-999 Leviant and Reichart (2015).151515http://leviants.com/ira.leviant/MultilingualVSMdata.html

Our results demonstrate similar patterns as for English, and indicate that our framework can be easily applied to other languages. For instance, the BEST-ADJ configuration (the same configuration as in Tab. 4 and Tab. 7) yields an improvement of 8 points and 4 points over the strongest adjectives baseline in IT and DE, respectively. We get similar improvements for nouns (IT: 3 points, DE: 2 points), and verbs (IT: 2, DE: 4).

TOEFL Evaluation

We also verify that the selection of class-specific configurations (Sect. 5.1) is useful beyond the core SimLex evaluation. For this aim, we evaluate on the A, V, and N TOEFL questions Landauer and Dumais (1997). The results are summarised in Tab. 7. Despite the limited size of the TOEFL dataset, we observe positive trends in the reported results (e.g., V-specific configurations yield a small gain on verb questions), showcasing the potential of class-specific training in this task.

6 Conclusion and Future Work

We have presented a novel framework for selecting class-specific context configurations which yield improved representations for prominent word classes: adjectives, verbs, and nouns. Its design and dependence on the Universal Dependencies annotation scheme makes it applicable in different languages. We have proposed an algorithm that is able to find a suitable class-specific configuration while making the search over the large space of possible context configurations computationally feasible. Each word class requires a different class-specific configuration to produce improved results on the class-specific subset of SimLex-999 in English, Italian, and German. We also show that the selection of context configurations is robust as once learned configuration may be effectively transferred to other data setups, tasks, and languages without additional retraining or fine-tuning.

In future work, we plan to test the framework with finer-grained contexts, investigating beyond POS-based word classes and dependency links. Exploring more sophisticated algorithms that can efficiently search richer configuration spaces is also an intriguing direction. Another research avenue is application of the context selection idea to other representation models beyond SGNS tested in this work, and experimenting with assigning weights to context subsets. Finally, we plan to test the portability of our approach to more languages.

Acknowledgments

This work is supported by the ERC Consolidator Grant LEXICAL: Lexical Acquisition Across Languages (no 648909). Roy Schwartz was supported by the Intel Collaborative Research Institute for Computational Intelligence (ICRI-CI). The authors are grateful to the anonymous reviewers for their helpful and constructive suggestions.

References