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Title: Beyond Word Importance:  Contextual Decomposition to Extract Interactions from LSTMs

Paper content:
The driving force behind the recent success of LSTMs has been their ability to learn complex and non-linear relationships.
Consequently, our inability to describe these relationships has led to LSTMs being characterized as black boxes.
To this end, we introduce contextual decomposition (CD), an interpretation algorithm for analysing individual predictions made by standard LSTMs, without any changes to the underlying model.
By decomposing the output of a LSTM, CD captures the contributions of combinations of words or variables to the final prediction of an LSTM.
On the task of sentiment analysis with the Yelp and SST data sets, we show that CD is able to reliably identify words and phrases of contrasting sentiment, and how they are combined to yield the LSTM's final prediction.
Using the phrase-level labels in SST, we also demonstrate that CD is able to successfully extract positive and negative negations from an LSTM, something which has not previously been done.
In comparison with simpler linear models, techniques from deep learning have achieved impressive accuracy by effectively learning non-linear interactions between features.
However, due to our inability to describe the learned interactions, this improvement in accuracy has come at the cost of state of the art predictive algorithms being commonly regarded as black-boxes.
In the domain of natural language processing (NLP), Long Short Term Memory networks (LSTMs) BID2 have become a basic building block, yielding excellent performance across a wide variety of tasks (Sutskever et al., 2014) BID10 BID7 , while remaining largely inscrutable.In this work, we introduce contextual decomposition (CD), a novel interpretation method for explaining individual predictions made by an LSTM without any modifications to the underlying model.
CD extracts information about not only which words contributed to a LSTM's prediction, but also how they were combined in order to yield the final prediction.
By mathematically decomposing the LSTM's output, we are able to disambiguate the contributions made at each step by different parts of the sentence.To validate the CD interpretations extracted from an LSTM, we evaluate on the problem of sentiment analysis.
In particular, we demonstrate that CD is capable of identifying words and phrases of differing sentiment within a given review.
CD is also used to successfully extract positive and negative negations from an LSTM, something that has not previously been done.
As a consequence of this analysis, we also show that prior interpretation methods produce scores which have document-level information built into them in complex, unspecified ways.
For instance, prior work often identifies strongly negative phrases contained within positive reviews as neutral, or even positive.
In this paper, we have proposed contextual decomposition (CD), an algorithm for interpreting individual predictions made by LSTMs without modifying the underlying model.
In both NLP and general applications of LSTMs, CD produces importance scores for words (single variables in general), phrases (several variables together) and word interactions (variable interactions).
Using two sentiment analysis datasets for empirical validation, we first show that for information also produced by prior methods, such as word-level scores, our method compares favorably.
More importantly, we then show that CD is capable of identifying phrases of varying sentiment, and extracting meaningful word (or variable) interactions.
This movement beyond word-level importance is critical for understanding a model as complex and highly non-linear as LSTMs.
6 APPENDIX Figure 4: Logistic regression coefficients versus coefficients extracted from an LSTM on SST.
We include a least squares regression line.
Stronger linear relationships in the plots correspond to better interpretation techniques.

assistant: text: We introduce contextual decompositions, an interpretation algorithm for LSTMs capable of extracting word, phrase and interaction-level importance score