Neural Word Embedding as Implicit Matrix Factorization
Neural Word Embedding
as Implicit Matrix Factorization
Omer Levy
Department of Computer Science
Bar-Ilan University
Yoav Goldberg
Department of Computer Science
Bar-Ilan University
yoav.
Abstract
We analyze skip-gram with negative-sampling (SGNS), a word embedding
method introduced by Mikolov et al., and show that it is implicitly factorizing
a word-context matrix, whose cells are the pointwise mutual information (PMI) of
the respective word and context pairs, shifted by a global constant. We find that
another embedding method, NCE, is implicitly factorizing a similar matrix, where
each cell is the (shifted) log conditional probability of a word given its context.
We show that using a sparse Shifted Positive PMI word-context matrix to represent
words improves results on two word similarity tasks and one of two analogy tasks.
When dense low-dimensional vectors are preferred, exact factorization with SVD
can achieve solutions that are at least as good as SGNS’s solutions for word simi-
larity tasks. On analogy questions SGNS remains superior to SVD. We conjecture
that this stems from the weighted nature of SGNS’s factorization.
1 Introduction
Most tasks in natural language processing and understanding involve looking at words, and could
benefit from word representations that do not treat individual words as unique symbols, but instead
reflect similarities and dissimilarities between them. The common paradigm for deriving such repre-
sentations is based on the distributional hypothesis of Harris [15], which states that words in similar
contexts have similar meanings. This has given rise to many word representation methods in the
NLP literature, the vast majority of whom can be described in terms of a word-context matrix M in
which each row i corresponds to a word, each column j to a context in which the word appeared, and
each matrix entry Mij corresponds to some association measure between the word and the context.
Words are then represented as rows in M or in a dimensionality-reduced matrix based on M .
Recently, there has been a surge of work proposing to represent words as dense vectors, derived using
various training methods inspired from neural-network language modeling [3, 9, 23, 21]. These
representations, referred to as “neural embeddings” or “word embeddings”, have been shown to
perform well in a variety of NLP tasks [26, 10, 1]. In particular, a sequence of papers by Mikolov and
colleagues [20, 21] culminated in the skip-gram with negative-sampling (SGNS) training method
which is both efficient to train and provides state-of-the-art results on various linguistic tasks. The
training method (as implemented in the word2vec software package) is highly popular, but not
well understood. While it is clear that the training objective follows the distributional hypothesis
– by trying to maximize the dot-product between the vectors of frequently occurring word-context
pairs, and minimize it for random word-context pairs – very little is known about the quantity being
optimized by the algorithm, or the reason it is expected to produce good word representations.
In this work, we aim to broaden the theoretical understanding of neural-inspired word embeddings.
Specifically, we cast SGNS’s training method as weighted matrix factorization, and show that its
objective is implicitly factorizing a shifted PMI matrix – the well-known word-context PMI matrix
from the word-similarity literature, shifted by a constant offset. A similar result holds also for the
1
NCE embedding method of Mnih and Kavukcuoglu [24]. While it is impractical to directly use the
very high-dimensional and dense shifted PMI matrix, we propose to approximate it with the positive
shifted PMI matrix (Shifted PPMI), which is sparse. Shifted PPMI is far better at optimizing SGNS’s
objective, and performs slightly better than word2vec derived vectors on several linguistic tasks.
Finally, we suggest a simple spectral algorithm that is based on performing SVD over the Shifted
PPMI matrix. The spectral algorithm outperforms both SGNS and the Shifted PPMI matrix on the
word similarity tasks, and is scalable to large corpora. However, it lags behind the SGNS-derived
representation on word-analogy tasks. We conjecture that this behavior is related to the fact that
SGNS performs weighted matrix factorization, giving more influence to frequent pairs, as opposed
to SVD, which gives the same weight to all matrix cells. While the weighted and non-weighted
objectives share the same optimal solution (perfect reconstruction of the shifted PMI matrix), they
result in different generalizations when combined with dimensionality constraints.
2 Background: Skip-Gram with Negative Sampling (SGNS)
Our departure point is SGNS – the skip-gram neural embedding model introduced in [20] trained
using the negative-sampling procedure presented in [21]. In what follows, we summarize the SGNS
model and introduce our notation. A detailed derivation of the SGNS model is available in [14].
Setting and Notation The skip-gram model assumes a corpus of words w ∈ VW and their
contexts c ∈ VC , where VW and VC are the word and context vocabularies. In [20, 21]
the words come from an unannotated textual corpus of words w1, w2, . . . , wn (typically n is in
the billions) and the contexts for word wi are the words surrounding it in an L-sized window
wi−L, . . . , wi−1, wi+1, . . . , wi+L. Other definitions of contexts are possible [18]. We denote the
collection of observed words and context pairs as D. We use #(w, c) to denote the number of times
the pair (w, c) appears in D. Similarly, #(w) =
∑
c′∈VC #(w, c
′) and #(c) =
∑
w′∈VW #(w
′, c)
are the number of times w and c occurred in D, respectively.
Each word w ∈ VW is associated with a vector ~w ∈ Rd and similarly each context c ∈ VC is
represented as a vector ~c ∈ Rd, where d is the embedding’s dimensionality. The entries in the
vectors are latent, and treated as parameters to be learned. We sometimes refer to the vectors ~w as
rows in a |VW |×d matrix W , and to the vectors ~c as rows in a |VC |×d matrix C. In such cases, Wi
(Ci) refers to the vector representation of the ith word (context) in the corresponding vocabulary.
When referring to embeddings produced by a specific method x, we will usually use W x and Cx
explicitly, but may use just W and C when the method is clear from the discussion.
SGNS’s Objective Consider a word-context pair (w, c). Did this pair come from the observed data
D? Let P (D = 1|w, c) be the probability that (w, c) came from the data, and P (D = 0|w, c) =
1− P (D = 1|w, c) the probability that (w, c) did not. The distribution is modeled as:
P (D = 1|w, c) = σ(~w · ~c) =
1
1 + e−~w·~c
where ~w and ~c (each a d-dimensional vector) are the model parameters to be learned.
The negative sampling objective tries to maximize P (D = 1|w, c) for observed (w, c) pairs while
maximizing P (D = 0|w, c) for randomly sampled “negative” examples (hence the name “negative
sampling”), under the assumption that randomly selecting a context for a given word is likely to
result in an unobserved (w, c) pair. SGNS’s objective for a single (w, c) observation is then:
log σ(~w · ~c) + k · EcN∼PD [log σ(−~w · ~cN )] (1)
where k is the number of “negative” samples and cN is the sampled context, drawn according to the
empirical unigram distribution PD(c) =
#(c)
|D| .
1
1In the algorithm described in [21], the negative contexts are sampled according to p3/4(c) = #c
3/4
Z
instead of the unigram distribution #c
Z
. Sampling according to p3/4 indeed produces somewhat superior results
on some of the semantic evaluation tasks. It is straight-forward to modify the PMI metric in a similar fashion
by replacing the p(c) term with p3/4(c), and doing so shows similar trends in the matrix-based methods as it
does in word2vec’s stochastic gradient based training method. We do not explore this further in this paper,
and report results using the unigram distribution.
2
The objective is trained in an online fashion using stochastic gradient updates over the observed
pairs in the corpus D. The global objective then sums over the observed (w, c) pairs in the corpus:
` =
∑
w∈VW
∑
c∈VC
#(w, c) (log σ(~w · ~c) + k · EcN∼PD [log σ(−~w · ~cN )]) (2)
Optimizing this objective makes observed word-context pairs have similar embeddings, while scat-
tering unobserved pairs. Intuitively, words that appear in similar contexts should have similar em-
beddings, though we are not familiar with a formal proof that SGNS does indeed maximize the
dot-product of similar words.
3 SGNS as Implicit Matrix Factorization
SGNS embeds both words and their contexts into a low-dimensional space Rd, resulting in the
word and context matrices W and C. The rows of matrix W are typically used in NLP tasks (such
as computing word similarities) while C is ignored. It is nonetheless instructive to consider the
product W · C> = M . Viewed this way, SGNS can be described as factorizing an implicit matrix
M of dimensions |VW | × |VC | into two smaller matrices.
Which matrix is being factorized? A matrix entry Mij corresponds to the dot product Wi · Cj =
~wi · ~cj . Thus, SGNS is factorizing a matrix in which each row corresponds to a word w ∈ VW ,
each column corresponds to a context c ∈ VC , and each cell contains a quantity f(w, c) reflecting
the strength of association between that particular word-context pair. Such word-context association
matrices are very common in the NLP and word-similarity literature, see e.g. [29, 2]. That said, the
objective of SGNS (equation 2) does not explicitly state what this association metric is. What can
we say about the association function f(w, c)? In other words, which matrix is SGNS factorizing?
3.1 Characterizing the Implicit Matrix
Consider the global objective (equation 2) above. For sufficiently large dimensionality d (i.e. allow-
ing for a perfect reconstruction of M ), each product ~w · ~c can assume a value independently of the
others. Under these conditions, we can treat the objective ` as a function of independent ~w ·~c terms,
and find the values of these terms that maximize it.
We begin by rewriting equation 2:
` =
∑
w∈VW
∑
c∈VC
#(w, c) (log σ(~w · ~c)) +
∑
w∈VW
∑
c∈VC
#(w, c) (k · EcN∼PD [log σ(−~w · ~cN )])
=
∑
w∈VW
∑
c∈VC
#(w, c) (log σ(~w · ~c)) +
∑
w∈VW
#(w) (k · EcN∼PD [log σ(−~w · ~cN )]) (3)
and explicitly expressing the expectation term:
EcN∼PD [log σ(−~w · ~cN )] =
∑
cN∈VC
#(cN )
|D|
log σ(−~w · ~cN )
=
#(c)
|D|
log σ(−~w · ~c) +
∑
cN∈VC\{c}
#(cN )
|D|
log σ(−~w · ~cN ) (4)
Combining equations 3 and 4 reveals the local objective for a specific (w, c) pair:
`(w, c) = #(w, c) log σ(~w · ~c) + k ·#(w) ·
#(c)
|D|
log σ(−~w · ~c) (5)
To optimize the objective, we define x = ~w · ~c and find its partial derivative with respect to x:
∂`
∂x
= #(w, c) · σ(−x)− k ·#(w) ·
#(c)
|D|
· σ(x)
We compare the derivative to zero, and after some simplification, arrive at:
e2x −
#(w, c)
k ·#(w) · #(c)|D|
− 1
ex − #(w, c)
k ·#(w) · #(c)|D|
= 0
3
If we define y = ex, this equation becomes a quadratic equation of y, which has two solutions,
y = −1 (which is invalid given the definition of y) and:
y =
#(w, c)
k ·#(w) · #(c)|D|
=
#(w, c) · |D|
#w ·#(c)
·
1
k
Substituting y with ex and x with ~w · ~c reveals:
~w · ~c = log
(
#(w, c) · |D|
#(w) ·#(c)
·
1
k
)
= log
(
#(w, c) · |D|
#(w) ·#(c)
)
− log k (6)
Interestingly, the expression log
(
#(w,c)·|D|
#(w)·#(c)
)
is the well-known pointwise mutual information
(PMI) of (w, c), which we discuss in depth below.
Finally, we can describe the matrix M that SGNS is factorizing:
MSGNSij = Wi · Cj = ~wi · ~cj = PMI(wi, cj)− log k (7)
For a negative-sampling value of k = 1, the SGNS objective is factorizing a word-context matrix
in which the association between a word and its context is measured by f(w, c) = PMI(w, c).
We refer to this matrix as the PMI matrix, MPMI . For negative-sampling values k > 1, SGNS is
factorizing a shifted PMI matrix MPMIk = MPMI − log k.
Other embedding methods can also be cast as factorizing implicit word-context matrices. Using a
similar derivation, it can be shown that noise-contrastive estimation (NCE) [24] is factorizing the
(shifted) log-conditional-probability matrix:
MNCEij = ~wi · ~cj = log
(
#(w, c)
#(c)
)
− log k = logP (w|c)− log k (8)
3.2 Weighted Matrix Factorization
We obtained that SGNS’s objective is optimized by setting ~w · ~c = PMI(w, c) − log k for every
(w, c) pair. However, this assumes that the dimensionality of ~w and ~c is high enough to allow for
perfect reconstruction. When perfect reconstruction is not possible, some ~w ·~c products must deviate
from their optimal values. Looking at the pair-specific objective (equation 5) reveals that the loss
for a pair (w, c) depends on its number of observations (#(w, c)) and expected negative samples
(k ·#(w) ·#(c)/|D|). SGNS’s objective can now be cast as a weighted matrix factorization prob-
lem, seeking the optimal d-dimensional factorization of the matrix MPMI − log k under a metric
which pays more for deviations on frequent (w, c) pairs than deviations on infrequent ones.
3.3 Pointwise Mutual Information
Pointwise mutual information is an information-theoretic association measure between a pair of
discrete outcomes x and y, defined as:
PMI(x, y) = log
P (x, y)
P (x)P (y)
(9)
In our case, PMI(w, c) measures the association between a word w and a context c by calculating
the log of the ratio between their joint probability (the frequency in which they occur together)
and their marginal probabilities (the frequency in which they occur independently). PMI can be
estimated empirically by considering the actual number of observations in a corpus:
PMI(w, c) = log
#(w, c) · |D|
#(w) ·#(c)
(10)
The use of PMI as a measure of association in NLP was introduced by Church and Hanks [8] and
widely adopted for word similarity tasks [11, 27, 29].
Working with the PMI matrix presents some computational challenges. The rows of MPMI con-
tain many entries of word-context pairs (w, c) that were never observed in the corpus, for which
4
PMI(w, c) = log 0 = −∞. Not only is the matrix ill-defined, it is also dense, which is a major
practical issue because of its huge dimensions |VW | × |VC |. One could smooth the probabilities
using, for instance, a Dirichlet prior by adding a small “fake” count to the underlying counts matrix,
rendering all word-context pairs observed. While the resulting matrix will not contain any infinite
values, it will remain dense.
An alternative approach, commonly used in NLP, is to replace the MPMI matrix with MPMI0 , in
which PMI(w, c) = 0 in cases #(w, c) = 0, resulting in a sparse matrix. We note that MPMI0 is
inconsistent, in the sense that observed but “bad” (uncorrelated) word-context pairs have a negative
matrix entry, while unobserved (hence worse) ones have 0 in their corresponding cell. Consider for
example a pair of relatively frequent words (high P (w) and P (c)) that occur only once together.
There is strong evidence that the words tend not to appear together, resulting in a negative PMI
value, and hence a negative matrix entry. On the other hand, a pair of frequent words (same P (w)
and P (c)) that is never observed occurring together in the corpus, will receive a value of 0.
A sparse and consistent alternative from the NLP literature is to use the positive PMI (PPMI) metric,
in which all negative values are replaced by 0:
PPMI(w, c) = max (PMI (w, c) , 0) (11)
When representing words, there is some intuition behind ignoring negative values: humans can
easily think of positive associations (e.g. “Canada” and “snow”) but find it much harder to invent
negative ones (“Canada” and “desert”). This suggests that the perceived similarity of two words
is more influenced by the positive context they share than by the negative context they share. It
therefore makes some intuitive sense to discard the negatively associated contexts and mark them
as “uninformative” (0) instead.2 Indeed, it was shown that the PPMI metric performs very well on
semantic similarity tasks [5].
BothMPMI0 andM
PPMI are well known to the NLP community. In particular, systematic comparisons
of various word-context association metrics show that PMI, and more so PPMI, provide the best
results for a wide range of word-similarity tasks [5, 16]. It is thus interesting that the PMI matrix
emerges as the optimal solution for SGNS’s objective.
4 Alternative Word Representations
As SGNS with k = 1 is attempting to implicitly factorize the familiar matrix MPMI, a natural algo-
rithm would be to use the rows of MPPMI directly when calculating word similarities. Though PPMI
is only an approximation of the original PMI matrix, it still brings the objective function very close
to its optimum (see Section 5.1). In this section, we propose two alternative word representations
that build upon MPPMI.
4.1 Shifted PPMI
While the PMI matrix emerges from SGNS with k = 1, it was shown that different values of k can
substantially improve the resulting embedding. With k > 1, the association metric in the implicitly
factorized matrix is PMI(w, c)− log(k). This suggests the use of Shifted PPMI (SPPMI), a novel
association metric which, to the best of our knowledge, was not explored in the NLP and word-
similarity communities:
SPPMIk(w, c) = max (PMI (w, c)− log k, 0) (12)
As with SGNS, certain values of k can improve the performance of MSPPMIk on different tasks.
4.2 Spectral Dimensionality Reduction: SVD over Shifted PPMI
While sparse vector representations work well, there are also advantages to working with dense low-
dimensional vectors, such as improved computational efficiency and, arguably, better generalization.
2A notable exception is the case of syntactic similarity. For example, all verbs share a very strong negative
association with being preceded by determiners, and past tense verbs have a very strong negative association to
be preceded by “be” verbs and modals.
5
An alternative matrix factorization method to SGNS’s stochastic gradient training is truncated Sin-
gular Value Decomposition (SVD) – a basic algorithm from linear algebra which is used to achieve
the optimal rank d factorization with respect to L2 loss [12]. SVD factorizes M into the product
of three matrices U · Σ · V >, where U and V are orthonormal and Σ is a diagonal matrix of sin-
gular values. Let Σd be the diagonal matrix formed from the top d singular values, and let Ud and
Vd be the matrices produced by selecting the corresponding columns from U and V . The matrix
Md = Ud ·Σd ·V >d is the matrix of rank d that best approximates the original matrixM , in the sense
that it minimizes the approximation errors. That is, Md = arg minRank(M ′)=d ‖M ′ −M‖2.
When using SVD, the dot-products between the rows of W = Ud ·Σd are equal to the dot-products
between rows of Md. In the context of word-context matrices, the dense, d dimensional rows of W
are perfect substitutes for the very high-dimensional rows of Md. Indeed another common approach
in the NLP literature is factorizing the PPMI matrix MPPMI with SVD, and then taking the rows
of W SVD = Ud · Σd and CSVD = Vd as word and context representations, respectively. However,
using the rows ofW SVD as word representations consistently under-perform theW SGNS embeddings
derived from SGNS when evaluated on semantic tasks.
Symmetric SVD We note that in the SVD-based factorization, the resulting word and context
matrices have very different properties. In particular, the context matrix CSVD is orthonormal while
the word matrix W SVD is not. On the other hand, the factorization achieved by SGNS’s training
procedure is much more “symmetric”, in the sense that neitherWW2V nor CW2V is orthonormal, and
no particular bias is given to either of the matrices in the training objective. We therefore propose
achieving similar symmetry with the following factorization:
W SVD1/2 = Ud ·
√
Σd C
SVD1/2 = Vd ·
√
Σd (13)
While it is not theoretically clear why the symmetric approach is better for semantic tasks, it does
work much better empirically.3
SVD versus SGNS The spectral algorithm has two computational advantages over stochastic gra-
dient training. First, it is exact, and does not require learning rates or hyper-parameter tuning.
Second, it can be easily trained on count-aggregated data (i.e. {(w, c,#(w, c))} triplets), making it
applicable to much larger corpora than SGNS’s training procedure, which requires each observation
of (w, c) to be presented separately.
On the other hand, the stochastic gradient method has advantages as well: in contrast to SVD, it
distinguishes between observed and unobserved events; SVD is known to suffer from unobserved
values [17], which are very common in word-context matrices. More importantly, SGNS’s objective
weighs different (w, c) pairs differently, preferring to assign correct values to frequent (w, c) pairs
while allowing more error for infrequent pairs (see Section 3.2). Unfortunately, exact weighted
SVD is a hard computational problem [25]. Finally, because SGNS cares only about observed
(and sampled) (w, c) pairs, it does not require the underlying matrix to be a sparse one, enabling
optimization of dense matrices, such as the exact PMI − log k matrix. The same is not feasible
when using SVD.
An interesting middle-ground between SGNS and SVD is the use of stochastic matrix factorization
(SMF) approaches, common in the collaborative filtering literature [17]. In contrast to SVD, the
SMF approaches are not exact, and do require hyper-parameter tuning. On the other hand, they
are better than SVD at handling unobserved values, and can integrate importance weighting for
examples, much like SGNS’s training procedure. However, like SVD and unlike SGNS’s procedure,
the SMF approaches work over aggregated (w, c) statistics allowing (w, c, f(w, c)) triplets as input,
making the optimization objective more direct, and scalable to significantly larger corpora. SMF
approaches have additional advantages over both SGNS and SVD, such as regularization, opening
the way to a range of possible improvements. We leave the exploration of SMF-based algorithms
for word embeddings to future work.
3The approach can be generalized toW SVDα = Ud ·(Σd)α, making α a tunable parameter. This observation
was previously made by Caron [7] and investigated in [6, 28], showing that different values of α indeed perform
better than others for various tasks. In particular, setting α = 0 performs well for many tasks. We do not explore
tuning the α parameter in this work.
6
Method PMI− log k SPPMI SVD SGNS
d = 100 d = 500 d = 1000 d = 100 d = 500 d = 1000
k = 1 0% 0.00009% 26.1% 25.2% 24.2% 31.4% 29.4% 7.40%
k = 5 0% 0.00004% 95.8% 95.1% 94.9% 39.3% 36.0% 7.13%
k = 15 0% 0.00002% 266% 266% 265% 7.80% 6.37% 5.97%
Table 1: Percentage of deviation from the optimal objective value (lower values are better). See 5.1 for details.
5 Empirical Results
We compare the matrix-based algorithms to SGNS in two aspects. First, we measure how well each
algorithm optimizes the objective, and then proceed to evaluate the methods on various linguistic
tasks. We find that for some tasks there is a large discrepancy between optimizing the objective and
doing well on the linguistic task.
Experimental Setup All models were trained on English Wikipedia, pre-processed by removing
non-textual elements, sentence splitting, and tokenization. The corpus contains 77.5 million sen-
tences, spanning 1.5 billion tokens. All models were derived using a window of 2 tokens to each
side of the focus word, ignoring words that appeared less than 100 times in the corpus, resulting
in vocabularies of 189,533 terms for both words and contexts. To train the SGNS models, we used
a modified version of word2vec which receives a sequence of pre-extracted word-context pairs
[18].4 We experimented with three values of k (number of negative samples in SGNS, shift param-
eter in PMI-based methods): 1, 5, 15. For SVD, we take W = Ud ·
√
Σd as explained in Section 4.
5.1 Optimizing the Objective
Now that we have an analytical solution for the objective, we can measure how well each algorithm
optimizes this objective in practice. To do so, we calculated `, the value of the objective (equation 2)
given each word (and context) representation.5 For sparse matrix representations, we substituted ~w·~c
with the matching cell’s value (e.g. for SPPMI, we set ~w · ~c = max(PMI(w, c) − log k, 0)). Each
algorithm’s ` value was compared to `Opt, the objective when setting ~w · ~c = PMI(w, c) − log k,
which was shown to be optimal (Section 3.1). The percentage of deviation from the optimum is
defined by (`− `Opt)/(`Opt) and presented in table 1.
We observe that SPPMI is indeed a near-perfect approximation of the optimal solution, even though
it discards a lot of information when considering only positive cells. We also note that for the
factorization methods, increasing the dimensionality enables better solutions, as expected. SVD is
slightly better than SGNS at optimizing the objective for d ≤ 500 and k = 1. However, while
SGNS is able to leverage higher dimensions and reduce its error significantly, SVD fails to do so.
Furthermore, SVD becomes very erroneous as k increases. We hypothesize that this is a result of the
increasing number of zero-cells, which may cause SVD to prefer a factorization that is very close to
the zero matrix, since SVD’s L2 objective is unweighted, and does not distinguish between observed
and unobserved matrix cells.
5.2 Performance of Word Representations on Linguistic Tasks
Linguistic Tasks and Datasets We evaluated the word representations on four dataset, covering
word similarity and relational analogy tasks. We used two datasets to evaluate pairwise word simi-
larity: Finkelstein et al.’s WordSim353 [13] and Bruni et al.’s MEN [4]. These datasets contain word
pairs together with human-assigned similarity scores. The word vectors are evaluated by ranking
the pairs according to their cosine similarities, and measuring the correlation (Spearman’s ρ) with
the human ratings.
The two analogy datasets present questions of the form “a is to a∗ as b is to b∗”, where b∗ is hidden,
and must be guessed from the entire vocabulary. The Syntactic dataset [22] contains 8000 morpho-
4http://www.bitbucket.org/yoavgo/word2vecf
5 Since it is computationally expensive to calculate the exact objective, we approximated it. First, instead of
enumerating every observed word-context pair in the corpus, we sampled 10 million such pairs, according to
their prevalence. Second, instead of calculating the expectation term explicitly (as in equation 4), we sampled
a negative example {(w, cN )} for each one of the 10 million “positive” examples, using the contexts’ unigram
distribution, as done by SGNS’s optimization procedure (explained in Section 2).
7
http://www.bitbucket.org/yoavgo/word2vecf
WS353 (WORDSIM) [13] MEN (WORDSIM) [4] MIXED ANALOGIES [20] SYNT. ANALOGIES [22]
Representation Corr. Representation Corr. Representation Acc. Representation Acc.
SVD (k=5) 0.691 SVD (k=1) 0.735 SPPMI (k=1) 0.655 SGNS (k=15) 0.627
SPPMI (k=15) 0.687 SVD (k=5) 0.734 SPPMI (k=5) 0.644 SGNS (k=5) 0.619
SPPMI (k=5) 0.670 SPPMI (k=5) 0.721 SGNS (k=15) 0.619 SGNS (k=1) 0.59
SGNS (k=15) 0.666 SPPMI (k=15) 0.719 SGNS (k=5) 0.616 SPPMI (k=5) 0.466
SVD (k=15) 0.661 SGNS (k=15) 0.716 SPPMI (k=15) 0.571 SVD (k=1) 0.448
SVD (k=1) 0.652 SGNS (k=5) 0.708 SVD (k=1) 0.567 SPPMI (k=1) 0.445
SGNS (k=5) 0.644 SVD (k=15) 0.694 SGNS (k=1) 0.540 SPPMI (k=15) 0.353
SGNS (k=1) 0.633 SGNS (k=1) 0.690 SVD (k=5) 0.472 SVD (k=5) 0.337
SPPMI (k=1) 0.605 SPPMI (k=1) 0.688 SVD (k=15) 0.341 SVD (k=15) 0.208
Table 2: A comparison of word representations on various linguistic tasks. The different representations were
created by three algorithms (SPPMI, SVD, SGNS) with d = 1000 and different values of k.
syntactic analogy questions, such as “good is to best as smart is to smartest”. The Mixed dataset [20]
contains 19544 questions, about half of the same kind as in Syntactic, and another half of a more se-
mantic nature, such as capital cities (“Paris is to France as Tokyo is to Japan”). After filtering ques-
tions involving out-of-vocabulary words, i.e. words that appeared in English Wikipedia less than
100 times, we remain with 7118 instances in Syntactic and 19258 instances in Mixed. The analogy
questions are answered using Levy and Goldberg’s similarity multiplication method [19], which is
state-of-the-art in analogy recovery: arg maxb∗∈VW \{a∗,b,a} cos(b
∗, a∗)·cos(b∗, b)/(cos(b∗, a)+ε).
The evaluation metric for the analogy questions is the percentage of questions for which the argmax
result was the correct answer (b∗).
Results Table 2 shows the experiments’ results. On the word similarity task, SPPMI yields better
results than SGNS, and SVD improves even more. However, the difference between the top PMI-
based method and the top SGNS configuration in each dataset is small, and it is reasonable to say
that they perform on-par. It is also evident that different values of k have a significant effect on all
methods: SGNS generally works better with higher values of k, whereas SPPMI and SVD prefer
lower values of k. This may be due to the fact that only positive values are retained, and high values
of k may cause too much loss of information. A similar observation was made for SGNS and SVD
when observing how well they optimized the objective (Section 5.1). Nevertheless, tuning k can
significantly increase the performance of SPPMI over the traditional PPMI configuration (k = 1).
The analogies task shows different behavior. First, SVD does not perform as well as SGNS and
SPPMI. More interestingly, in the syntactic analogies dataset, SGNS significantly outperforms the
rest. This trend is even more pronounced when using the additive analogy recovery method [22] (not
shown). Linguistically speaking, the syntactic analogies dataset is quite different from the rest, since
it relies more on contextual information from common words such as determiners (“the”, “each”,
“many”) and auxiliary verbs (“will”, “had”) to solve correctly. We conjecture that SGNS performs
better on this task because its training procedure gives more influence to frequent pairs, as opposed
to SVD’s objective, which gives the same weight to all matrix cells (see Section 3.2).
6 Conclusion
We analyzed the SGNS word embedding algorithms, and showed that it is implicitly factorizing the
(shifted) word-context PMI matrix MPMI − log k using per-observation stochastic gradient updates.
We presented SPPMI, a modification of PPMI inspired by our theoretical findings. Indeed, using
SPPMI can improve upon the traditional PPMI matrix. Though SPPMI provides a far better solution
to SGNS’s objective, it does not necessarily perform better than SGNS on linguistic tasks, as evident
with syntactic analogies. We suspect that this may be related to SGNS down-weighting rare words,
which PMI-based methods are known to exaggerate.
We also experimented with an alternative matrix factorization method, SVD. Although SVD was
relatively poor at optimizing SGNS’s objective, it performed slightly better than the other methods
on word similarity datasets. However, SVD underperforms on the word-analogy task. One of the
main differences between the SVD and SGNS is that SGNS performs weighted matrix factoriza-
tion, which may be giving it an edge in the analogy task. As future work we suggest investigating
weighted matrix factorizations of word-context matrices with PMI-based association metrics.
Acknowledgements This work was partially supported by the EC-funded project EXCITEMENT
(FP7ICT-287923). We thank Ido Dagan and Peter Turney for their valuable insights.
8
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