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Title: Can I Trust the Explainer? Verifying Post-Hoc Explanatory Methods

Paper content:
For AI systems to garner widespread public acceptance, we must develop methods capable of explaining the decisions of black-box models such as neural networks.
In this work, we identify two issues of current explanatory methods.
First, we show that two prevalent perspectives on explanations—feature-additivity and feature-selection—lead to fundamentally different instance-wise explanations.
In the literature, explainers from different perspectives are currently being directly compared, despite their distinct explanation goals.
The second issue is that current post-hoc explainers have only been thoroughly validated on simple models, such as linear regression, and, when applied to real-world neural networks, explainers are commonly evaluated under the assumption that the learned models behave reasonably.
However, neural networks often rely on unreasonable correlations, even when producing correct decisions.
We introduce a verification framework for explanatory methods under the feature-selection perspective.
Our framework is based on a non-trivial neural network architecture trained on a real-world task, and for which we are able to provide guarantees on its inner workings.
We validate the efficacy of our evaluation by showing the failure modes of current explainers.
We aim for this framework to provide a publicly available,1 off-the-shelf evaluation when the feature-selection perspective on explanations is needed.
A large number of post-hoc explanatory methods have recently been developed with the goal of shedding light on highly accurate, yet black-box machine learning models (Ribeiro et al., 2016a; Lundberg & Lee, 2017; Arras et al., 2017; Shrikumar et al., 2017; Ribeiro et al., 2016b; Plumb et al., 2018; Chen et al., 2018) .
Among these methods, there are currently at least two widely used perspectives on explanations: feature-additivity (Ribeiro et al., 2016a; Lundberg & Lee, 2017; Shrikumar et al., 2017; Arras et al., 2017) and feature-selection (Chen et al., 2018; Ribeiro et al., 2018; Carter et al., 2018) , which we describe in detail in the sections below.
While both shed light on the overall behavior of a model, we show that, when it comes to explaining the prediction on a single input in isolation, i.e., instance-wise explanations, the two perspectives lead to fundamentally different explanations.
In practice, explanatory methods adhering to different perspectives are being directly compared.
For example, Chen et al. (2018) and Yoon et al. (2019) compare L2X, a feature-selection explainer, with LIME (Ribeiro et al., 2016a) and SHAP (Lundberg & Lee, 2017) , two feature-additivity explainers.
We draw attention to the fact that these comparisons may not be coherent, given the fundamentally different explanation targets, and we discuss the strengths and limitations of the two perspectives.
Secondly, while current explanatory methods are successful in pointing out catastrophic biases, such as relying on headers to discriminate between pieces of text about Christianity and atheism (Ribeiro et al., 2016a) , it is an open question to what extent they are reliable when the model that they aim to explain (which we call the target model) has a less dramatic bias.
This is a difficult task, precisely because the ground-truth decision-making process of neural networks is not known.
Consequently, when applied to complex neural networks trained on real-world datasets, a prevalent way to evaluate the explainers is to assume that the target models behave reasonably, i.e., that they did not rely on irrelevant correlations.
For example, in their morphosyntactic agreement paradigm, Pörner et al. (2018) assume that a model that predicts if a verb should be singular or plural given the tokens before the verb, must be doing so by focusing on a noun that the model had identified as the subject.
Such assumptions may be poor, since recent works show a series of surprising spurious correlations in human-annotated datasets, on which neural networks learn to heavily rely (Gururangan et al., 2018; Glockner et al., 2018; Carmona et al., 2018) .
Therefore, it is not reliable to penalize an explainer for pointing to tokens that just do not appear significant to us.
We address the above issue by proposing a framework capable of generating evaluation tests for the explanatory methods under the feature-selection perspective.
Our tests consist of pairs of (target model, dataset).
Given a pair, for each instance in the dataset, the specific architecture of our model allows us to identify a subset of tokens that have zero contribution to the model's prediction on the instance.
We further identify a subset of tokens clearly relevant to the prediction.
Hence, we test if explainers rank zero-contribution tokens higher than relevant tokens.
We instantiated our framework on three pairs of (target model, dataset) on the task of multi-aspect sentiment analysis.
Each pair corresponds to an aspect and the three models (of same architecture) have been trained independently.
We highlight that our test is not a sufficient test for concluding the power of explainers in full generality, since we do not know the whole ground-truth behaviour of the target models.
Indeed, we do not introduce an explanation generation framework but a framework for generating evaluation tests for which we provide certain guarantees on the behaviour of the target model.
Under these guarantees we are able to test the explainers for critical failures.
Our framework therefore generates necessary evaluation tests, and our metrics penalize explainers only when we are able to guarantee that they produced an error.
To our knowledge, we are the first to introduce an automatic and non-trivial evaluation test that does not rely on speculations on the behavior of the target model.
Finally, we evaluate L2X (Chen et al., 2018) , a feature-selection explainer, under our test.
Even though our test is specifically designed for feature-selection explanatory methods, since, in practice, the two types of explainers are being compared, and, since LIME (Ribeiro et al., 2016a) and SHAP (Lundberg & Lee, 2017) are two very popular explainers, we were interested in how the latter perform on our test, even though they adhere to the feature-additivity perspective.
Interestingly, we find that, most of the time, LIME and SHAP perform better than L2X.
We will detail in Section 5 the reasons why we believe this is the case.
We provide the error rates of these explanatory methods to raise awareness of their possible modes of failure under the feature-selection perspective of explanations.
For example, our findings show that, in certain cases, the explainers predict the most relevant token to be among the tokens with zero contribution.
We will release our test, which can be used off-the-shelf, and encourage the community to use it for testing future work on explanatory methods under the feature-selection perspective.
We also note that our methodology for creating this evaluation is generic and can be instantiated on other tasks or areas of research.
In this work, we instantiate our framework on the RCNN model trained on the BeerAdvocate corpus, 3 on which the RCNN was initially evaluated (Lei et al., 2016) .
BeerAdvocate consists of a total of ≈ .100K human-generated multi-aspect beer reviews, where the three considered aspects are appearance, aroma, and palate.
The reviews are accompanied with fractional ratings originally between 0 and 5 for each aspect independently.
The RCNN is a regression model with the goal to predict the rating, rescaled between 0 and 1 for simplicity.
Three separate RCNNs are trained, one for each aspect independently, with the same default settings.
4 With the above procedure, we gathered three datasets D a , one for each aspect a.
For each dataset, we know that for each instance x ∈ D a , the set of non-selected tokens N x has zero contribution to the prediction of the model.
For obtaining the clearly relevant tokens, we chose a threshold of τ = 0.1, since the scores are in [0, 1], and the ground-truth ratings correspond to {0, 0.1, 0.2, . . . , 1}.
Therefore, a change in prediction of 0.1 is to be considered clearly significant for this task.
We provide several statistics of our datasets in Appendix A. For example, we provide the average lengths of the reviews, of the selected tokens per review, of the clearly relevant tokens among the selected, and of the non-selected tokens.
We note that we usually obtained 1 or 2 clearly relevant tokens per datapoints, showing that our threshold of 0.1 is likely very strict.
However, we prefer to be more conservative in order to ensure high guarantees on our evaluation test.
We also provide the percentages of datapoints eliminated in order to ensure the no-handshake condition (Equation 7).
Evaluating explainers.
We test three popular explainers: LIME (Ribeiro et al., 2016a), SHAP (Lundberg & Lee, 2017) , and L2X (Chen et al., 2018) .
We used the code of the explainers as provided in the original repositories, 5 with their default settings for text explanations, with the exception that, for L2X, we set the dimension of the word embeddings to 200 (the same as in the RCNN), and we also allowed training for a maximum of 30 epochs instead of 5.
As mentioned in Section 3, LIME and SHAP adhere to the feature-additivity perspective, hence our evaluation is not directly targeting these explainers.
However, we see in Table 1 that, in practice, LIME and SHAP outperformed L2X on the majority of the metrics, even though L2X is a featureselection explainer.
We hypothesize that a major limitation of L2X is the requirement to know the number of important features per instance.
Indeed, L2X learns a distribution over the set of features by maximizing the mutual information between subsets of K features and the response variable, where K is assumed to be known.
In practice, one usually does not know how many features per instance a model relied on.
To test L2X under real-world circumstances, we used as K the average number of tokens highlighted by human annotators on the subset manually annotated by McAuley et al. (2012) .
We obtained an average K of 23, 18, and 13 for the three aspects, respectively.
In Table 1 , we see that, on metric (A), all explainers are prone to stating that the most relevant feature is a token with zero contribution, as much as 14.79% of the time for LIME and 12.95% of the time for L2X in the aroma aspect.
We consider this the most dramatic form of failure.
Metric (B) shows that both explainers can rank at least one zero-contribution token higher than a clearly relevant feature, i.e., there is at least one mistake in the predicted ranking.
Finally, metric (C) shows that, in average, SHAP only places one zero-contribution token ahead of a clearly relevant token for the first two aspects and around 9 tokens for the third aspect, while L2X places around 3-4 zero-contribution tokens ahead of a clearly relevant one for all three aspects.
Figure 4 : Explainers' rankings (with top 5 features on the right-hand side) on an instance from the palate aspect in our evaluation dataset.
Qualitative Analysis.
In Figure 6 , we present an example from our dataset of the palate aspect.
More examples in Appendix C. The heatmap corresponds to the ranking determined by each explainer, and the intensity of the color decreases linearly with the ranking of the tokens.
6 We only show in the heatmap the first K = 10 ranked tokens, for visibility reasons.
Tokens in S x are in bold, and the clearly relevant tokens from SR x are additionally underlined.
The first selected by the explainer is marked wth a rectangular.
Additionally the 5 ranks tokens by each explainer are on the right-hand side.
Firstly, we notice that both explainers are prone to attributing importance to nonselected tokens, with LIME and SHAP even ranking the tokens "mouthfeel" and "lacing" belonging to N x as first two (most important).
Further, "gorgeous", the only relevant word used by the model, did not even make it in top 13 tokens for L2X.
Instead, L2X gives "taste", "great", "mouthfeel" and "lacing" as most important tokens.
We note that if the explainer was evaluated by humans assuming that the RCNN behaves reasonably, then this choice could have well been considered correct.
In this work, we first shed light on an important distinction between two widely used perspectives of explanations.
Secondly, we introduced an off-the-shelf evaluation test for post-hoc explanatory methods under the feature-selection perspective.
To our knowledge, this is the first automatic verification framework offering guarantees on the behaviour of a non-trivial real-world neural network.
We presented the error rates on different metrics for three popular explanatory methods to raise awareness of the types of failures that these explainers can produce, such as incorrectly predicting even the most relevant token.
While instantiated on a natural language processing task, our methodology is generic and can be adapted to other tasks and other areas.
For example, in computer vision, one could train a neural network that first makes a hard selection of super-pixels to retain, and subsequently makes a prediction based on the image where the non-selected super-pixels have been blurred.
The same procedure of checking for zero contribution of non-selected super-pixels would then apply.
We also point out that the core algorithm in the majority of the current post-hoc explainers are also domain-agnostic.
Therefore, we expect our evaluation to provide a representative view of the fundamental limitations of the explainers.

assistant: text: An evaluation framework based on a real-world neural network for post-hoc explanatory methods