Communications of the ACM

Review articles

The Challenge of Crafting Intelligible Intelligence

By Daniel S. Weld, Gagan Bansal
Communications of the ACM, June 2019, Vol. 62 No. 6, Pages 70-79
10.1145/3282486
Comments (1)

Artificial Intelligence (AI) systems have reached or exceeded human performance for many circumscribed tasks. As a result, they are increasingly deployed in mission-critical roles, such as credit scoring, predicting if a bail candidate will commit another crime, selecting the news we read on social networks, and self-driving cars. Unlike other mission-critical software, extraordinarily complex AI systems are difficult to test: AI decisions are context specific and often based on thousands or millions of factors. Typically, AI behaviors are generated by searching vast action spaces or learned by the opaque optimization of mammoth neural networks operating over prodigious amounts of training data. Almost by definition, no clear-cut method can accomplish these AI tasks.

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Key Insights

Unfortunately, much AI-produced behavior is alien, that is, it can fail in unexpected ways. This lesson is most clearly seen in the performance of the latest deep neural network image analysis systems. While their accuracy at object-recognition on naturally occurring pictures is extraordinary, imperceptible changes to input images can lead to erratic predictions, as shown in Figure 1. Why are these recognition systems so brittle, making different predictions for apparently identical images? Unintelligible behavior is not limited to machine learning; many AI programs, such as automated planning algorithms, perform search-based look ahead and inference whose complexity exceeds human abilities to verify. While some search and planning algorithms are provably complete and optimal, intelligibility is still important, because the underlying primitives (for example, search operators or action descriptions) are usually approximations.²⁹ One can't trust a proof that is based on (possibly) incorrect premises.

Figure 1. Adding an imperceptibly small vector to an image changes the GoogLeNet³⁹ image recognizer's classification of the image from "panda" to "gibbon." Source: Goodfellow et al.⁹

Despite intelligibility's apparent value, it remains remarkably difficult to specify what makes a system "intelligible." (We discuss desiderata for intelligible behavior later in this article.) In brief, we seek AI systems where it is clear what factors caused the system's action,²⁴ allowing the users to predict how changes to the situation would have led to alternative behaviors, and permits effective control of the AI by enabling interaction. As we will illustrate, there is a central tension between a concise explanation and an accurate one.

As shown in Figure 2, our survey focuses on two high-level approaches to building intelligible AI software: ensuring the underlying reasoning or learned model is inherently interpretable, for example, by learning a linear model over a small number of well-understood features, and if it is necessary to use an inscrutable model, such as complex neural networks or deep-look ahead search, then mapping this complex system to a simpler, explanatory model for understanding and control.²⁸ Using an interpretable model provides the benefit of transparency and veracity; in theory, a user can see exactly what the model is doing. Unfortunately, interpretable methods may not perform as well as more complex ones, such as deep neural networks. Conversely, the approach of mapping to an explanatory model can apply to whichever AI technique is currently delivering the best performance, but its explanation inherently differs from the way the AI system actually operates. This yields a central conundrum: How can a user trust that such an explanation reflects the essence of the underlying decision and does not conceal important details? We posit the answer is to make the explanation system interactive so users can drill down until they are satisfied with their understanding.

Figure 2. Approaches for crafting intelligible AI.

The key challenge for designing intelligible AI is communicating a complex computational process to a human. This requires interdisciplinary skills, including HCI as well as AI and machine learning expertise. Furthermore, since the nature of explanation has long been studied by philosophy and psychology, these fields should also be consulted.

This article highlights key approaches and challenges for building intelligible intelligence, characterizes intelligibility, and explains why it is important even in systems with measurably high performance. We describe the benefits and limitations of GA²M—a powerful class of interpretable ML models. Then, we characterize methods for handling inscrutable models, discussing different strategies for mapping to a simpler, intelligible model appropriate for explanation and control. We sketch a vision for building interactive explanation systems, where the mapping changes in response to the user's needs. Lastly, we argue that intelligibility is important for search-based AI systems as well as for those based on machine learning and that similar solutions may be applied.

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Why Intelligibility Matters

While it has been argued that explanations are much less important than sheer performance in AI systems, there are many reasons why intelligibility is important. We start by discussing technical reasons, but social factors are important as well.

AI may have the wrong objective. In some situations, even 100% perfect performance may be insufficient, for example, if the performance metric is flawed or incomplete due to the difficulty of specifying it explicitly. Pundits have warned that an automated factory charged with maximizing paperclip production, could subgoal on killing humans, who are using resources that could otherwise be used in its task. While this example may be fanciful, it illustrates that it is remarkably difficult to balance multiple attributes of a utility function. For example, as Lipton observed,²⁵ "An algorithm for making hiring decisions should simultaneously optimize for productivity, ethics and legality." However, how does one express this trade-off? Other examples include balancing training error while uncovering causality in medicine and balancing accuracy and fairness in recidivism prediction.¹² For the latter, a simplified objective function such as accuracy combined with historically biased training data may cause uneven performance for different groups (for example, people of color). Intelligibility empowers users to determine if an AI is right for the right reasons.

AI may be using inadequate features. Features are often correlated, and when one feature is included in a model, machine learning algorithms extract as much signal as possible from it, indirectly modeling other features that were not included. This can lead to problematic models, as illustrated by Figure 4b (and described later), where the ML determined that a patient's prior history of asthma (a lung disease) was negatively correlated with death by pneumonia, presumably due to correlation with (unmodeled) variables, such as these patients receiving timely and aggressive therapy for lung problems. An intelligible model helps humans to spot these issues and correct them, for example, by adding additional features.⁴

Distributional drift. A deployed model may perform poorly in the wild, that is, when a difference exists between the distribution which was used during training and that encountered during deployment. Furthermore, the deployment distribution may change over time, perhaps due to feedback from the act of deployment. This is common in adversarial domains, such as spam detection, online ad pricing, and search engine optimization. Intelligibility helps users determine when models are failing to generalize.

Facilitating user control. Many AI systems induce user preferences from their actions. For example, adaptive news feeds predict which stories are likely most interesting to a user. As robots become more common and enter the home, preference learning will become ever more common. If users understand why the AI performed an undesired action, they can better issue instructions that will lead to improved future behavior.

User acceptance. Even if they do not seek to change system behavior, users have been shown to be happier with and more likely to accept algorithmic decisions if they are accompanied by an explanation.¹⁸ After being told they should have their kidney removed, it's natural for a patient to ask the doctor why—even if they don't fully understand the answer.

Improving human insight. While improved AI allows automation of tasks previously performed by humans, this is not their only use. In addition, scientists use machine learning to get insight from big data. Medicine offers several examples.⁴ Similarly, the behavior of AlphaGo³⁵ has revolutionized human understanding of the game. Intelligible models greatly facilitate these processes.

Legal imperatives. The European Union's GDPR legislation decrees citizens' right to an explanation, and other nations may follow. Furthermore, assessing legal liability is a growing area of concern; a deployed model (for example, self-driving cars) may introduce new areas of liability by causing accidents unexpected from a human operator, shown as "AI-specific error" in Figure 3. Auditing such situations to assess liability requires understanding the model's decisions.

Figure 3. The dashed blue shape indicates the space of possible mistakes humans can make.

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Defining Intelligibility

So far we have treated intelligibility informally. Indeed, few computing researchers have tried to formally define what makes an AI system interpretable, transparent, or intelligible,⁶ but one suggested criterion is human simulatability:²⁵ Can a human user easily predict the model's output for a given input? By this definition, sparse linear models are more interpretable than dense or non-linear ones.

Philosophers, such as Hempel and Salmon, have long debated the nature of explanation. Lewis²³ summarizes: "To explain an event is to provide some information about its causal history." But many causal explanations may exist. The fact that event C causes E is best understood relative to an imagined counterfactual scenario, where absent C, E would not have occurred; furthermore, C should be minimal, an intuition known to early scientists, such as William of Occam, and formalized by Halpern and Pearl.¹¹

Following this logic, we suggest a better criterion than simulatability is the ability to answer counterfactuals, aka "what-if" questions. Specifically, we say that a model is intelligible to the degree that a human user can predict how a change to a feature, for example, a small increase to its value, will change the model's output and if they can reliably modify that response curve. Note that if one can simulate the model, predicting its output, then one can predict the effect of a change, but not vice versa.

Linear models are especially interpretable under this definition because they allow the answering of counter-factuals. For example, consider a naive Bayes unigram model for sentiment analysis, whose objective is to predict the emotional polarity (positive or negative) of a textual passage. Even if the model were large, combining evidence from the presence of thousands of words, one could see the effect of a given word by looking at the sign and magnitude of the corresponding weight. This answers the question, "What if the word had been omitted?" Similarly, by comparing the weights associated with two words, one could predict the effect on the model of substituting one for the other.

Ranking intelligible models. Since one may have a choice of intelligible models, it is useful to consider what makes one preferable to another. Social science research suggests an explanation is best considered a social process, a conversation between explainer and explainee.^15,30 As a result, Grice's rules for cooperative communication¹⁰ may hold for intelligible explanations. Grice's maxim of quality says be truthful, only relating things that are supported by evidence. The maxim of quantity says to give as much information as is needed, and no more. The maxim of relation: only say things that are relevant to the discussion. The maxim of manner says to avoid ambiguity, being as clear as possible.

Miller summarizes decades of work by psychological research, noting that explanations are contrastive, that is, of the form "Why P rather than Q?" The event in question, P, is termed the fact and Q is called the foil.³⁰ Often the foil is not explicitly stated even though it is crucially important to the explanation process. For example, consider the question, "Why did you predict the image depicts an indigo bunting?" An explanation that points to the color blue implicitly assumes the foil is another bird, such as a chickadee. But perhaps the questioner wonders why the recognizer did not predict a pair of denim pants; in this case a more precise explanation might highlight the presence of wings and a beak. Clearly, an explanation targeted to the wrong foil will be unsatisfying, but the nature and sophistication of a foil can depend on the end user's expertise; hence, the ideal explanation will differ for different people.⁶ For example, to verify that an ML system is fair, an ethicist might generate more complex foils than a data scientist. Most ML explanation systems have restricted their attention to elucidating the behavior of a binary classifier, that is, where there is only one possible foil choice. However, as we seek to explain multiclass systems, addressing this issue becomes essential.

Many systems are simply too complex to understand without approximation. Here, the key challenge is deciding which details to omit. After many years of study, psychologists determined that several criteria can be prioritized for inclusion in an explanation: necessary causes (vs. sufficient ones); intentional actions (vs. those taken without deliberation); proximal causes (vs. distant ones); details that distinguish between fact and foil; and abnormal features.³⁰

According to Lombrozo, humans prefer explanations that are simpler (that is, contain fewer clauses), more general, and coherent (that is, consistent with what the human's prior beliefs).²⁶ In particular, she observed the surprising result that humans preferred simple (one clause) explanations to conjunctive ones, even when the probability of the latter was higher than the former.²⁶ These results raise interesting questions about the purpose of explanations in an AI system. Is an explanation's primary purpose to convince a human to accept the computer's conclusions (perhaps by presenting a simple, plausible, but unlikely explanation) or is it to educate the human about the most likely true situation? Tversky, Kahneman, and other psychologists have documented many cognitive biases that lead humans to incorrect conclusions; for example, people reason incorrectly about the probability of conjunctions, with a concrete and vivid scenario deemed more likely than an abstract one that strictly subsumes it.¹⁶ Should an explanation system exploit human limitations or seek to protect us from them?

Other studies raise an additional complication about how to communicate a system's uncertain predictions to human users. Koehler found that simply presenting an explanation for a proposition makes people think that it is more likely to be true.¹⁸ Furthermore, explaining a fact in the same way as previous facts have been explained amplifies this effect.³⁶

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Inherently Intelligible Models

Several AI systems are inherently intelligible, and we previously observed that linear models support counterfactual reasoning. Unfortunately, linear models have limited utility because they often result in poor accuracy. More expressive choices may include simple decision trees and compact decision lists. To concretely illustrate the benefits of intelligibility, we focus on Generalized additive models (GAMs), which are a powerful class of ML models that relate a set of features to the target using a linear combination of (potentially nonlinear) single-feature models called shape functions.²⁷ For example, if y represents the target and {x₁, .... x_n} represents the features, then a GAM model takes the form y = β₀ + Σ(x_j), where the f_is denote shape functions and the target y is computed by summing single-feature terms. Popular shape functions include non-linear functions such as splines and decision trees. With linear shape functions GAMs reduce to a linear models. GA²M models extend GAM models by including terms for pairwise interactions between features:

Caruana et al. observed that for domains containing a moderate number of semantic features, GA²M models achieve performance that is competitive with inscrutable models, such as random forests and neural networks, while remaining intelligible.⁴ Lou et al. observed that among methods available for learning GA²M models, the version with bagged shallow regression tree shape functions learned via gradient boosting achieves the highest accuracy.²⁷

Both GAM and GA²M are considered interpretable because the model's learned behavior can be easily understood by examining or visualizing the contribution of terms (individual or pairs of features) to the final prediction. For example, Figure 4 depicts a GA²M model trained to predict a patient's risk of dying due to pneumonia, showing the contribution (log odds) to total risk for a subset of terms. A positive contribution increases risk, whereas a negative contribution decreases risk. For example, Figure 4a shows how the patient's age affects predicted risk. While the risk is low and steady for young patients (for example, age < 20), it increases rapidly for older patients (age > 67). Interestingly, the model shows a sudden increase at age 86; perhaps a result of less aggressive care by doctors for patients "whose time has come." Even more surprising is the sudden drop for patients over 100. This might be another social effect; once a patient reaches the magic "100," he or she gets more aggressive care. One benefit of an interpretable model is its ability to highlight these issues, spurring deeper analysis.

Figure 4. A part of Figure 1 from Caruana et al.⁴ showing three (of 56 total) components for a GA²M model, which was trained to predict a patient's risk of dying from pneumonia.

Figure 4b illustrates another surprising aspect of the learned model; apparently, a history of asthma, a respiratory disease, decreases the patients risk of dying from pneumonia! This finding is counterintuitive to any physician, who recognizes that asthma, in fact, should in theory increase such risk. When Caruana et al. checked the data, they concluded the lower risk was likely due to correlated variables—asthma patients typically receive timely and aggressive therapy for lung issues. Therefore, although the model was highly accurate on the test set, it would likely fail, dramatically underestimating the risk to a patient with asthma who had not been previously treated for the disease.

Facilitating human control of GA²M models. A domain expert can fix such erroneous patterns learned by the model by setting the weight of the asthma term to zero. In fact, GA²Ms let users provide much more comprehensive feedback to the model by using a GUI to redraw a line graph for model terms.⁴ An alternative remedy might be to introduce a new feature to the model, representing whether the patient had been recently seen by a pulmonologist. After adding this feature, which is highly correlated with asthma, and retraining, the newly learned model would likely reflect that asthma (by itself) increases the risk of dying from pneumonia.

There are two more takeaways from this anecdote. First, the absence of an important feature in the data representation can cause any AI system to learn unintuitive behavior for another, correlated feature. Second, if the learner is intelligible, then this unintuitive behavior is immediately apparent, allowing appropriate skepticism (despite high test accuracy) and easier debugging.

Recall that GA²Ms are more expressive than simple GAMs because they include pairwise terms. Figure 4c depicts such a term for the features age and cancer. This explanation indicates that among the patients who have cancer, the younger ones are at higher risk. This may be because the younger patients who develop cancer are probably critically ill. Again, since doctors can readily inspect these terms, they know if the learner develops unexpected conclusions.

Limitations. As described, GA²M models are restricted to binary classification, and so explanations are clearly contrastive—there is only one choice of foil. One could extend GA²M to handle multiple classes by training n one-vs-rest classifiers or building a hierarchy of classifiers. However, while these approaches would yield a working multi-class classifier, we don't know if they preserve model intelligibility, nor whether a user could effectively adjust such a model by editing the shape functions.

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The key challenge for designing intelligible AI is communicating a complex computational process to a human.

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Furthermore, recall that GA²Ms decompose their prediction into effects of individual terms, which can be visualized. However, if users are confused about what terms mean, they will not understand the model or be able to ask meaningful "what-if" questions. Moreover, if there are too many features, the model's complexity may be overwhelming. Lipton notes that the effort required to simulate some models (such as decision trees) may grow logarithmically with the number of parameters,²⁵ but for GA²M the number of visualizations to inspect could increase quadratically. Several methods might help users manage this complexity; for example, the terms could be ordered by importance; however, it's not clear how to estimate importance. Possible methods include using an ablation analysis to compute influence of terms on model performance or computing the maximum contribution of terms as seen in the training samples. Alternatively, a domain expert could group terms semantically to facilitate perusal.

However, when the number of features grows into the millions—which occur when dealing with classifiers over text, audio, image, and video data—existing intelligible models do not perform nearly as well as inscrutable methods, like deep neural networks. Since these models combine millions of features in complex, nonlinear ways, they are beyond human capacity to simulate.

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Understanding Inscrutable Models

There are two ways that an AI model may be inscrutable. It may be provided as a blackbox API, such as Microsoft Cognitive Services, which uses machine learning to provide image-recognition capabilities but does not allow inspection of the underlying model. Alternatively, the model may be under the user's control yet extremely complex, such as a deep, neural network, where a user has access to myriad learned parameters but cannot reasonably interpret them. How can one best explain such models to the user?

The comprehensibility/fidelity tradeoff. A good explanation of an event is both easy to understand and faithful, conveying the true cause of the event. Unfortunately, these two criteria almost always conflict. Consider the predictions of a deep neural network with millions of nodes: a complete and accurate trace of the network's prediction would be far too complex to understand, but any simplification sacrifices faithfulness.

Finding a satisfying explanation, therefore, requires balancing the competing goals of comprehensibility and fidelity. Lakkaraju et al.²² suggest formulating an explicit optimization of this form and propose an approximation algorithm for generating global explanations in the form of compact sets of if-then rules. Ribeiro et al. describe a similar optimization algorithm that balances faithfulness and coverage in its search for summary rules.³⁴

Indeed, all methods for rendering an inscrutable model intelligible require mapping the complex model to a simpler one.²⁸ Several high-level approaches to mapping have been proposed.

Local explanations. One way to simplify the explanation of a learned model is to make it relative to a single input query. Such explanations, which are termed local³³ or instance-based,²² are akin to a doctor explaining specific reasons for a patient's diagnosis rather than communicating all of her medical knowledge. Contrast this approach with the global understanding of the model that one gets with a GA²M model. Mathematically, one can see a local explanation as currying—several variables in the model are fixed to specific values, allowing simplification.

Generating a local explanation is a common practice in AI systems. For example, early rule-based expert systems included explanation systems that augmented a trace of the system's reasoning—for a particular case—with background knowledge.³⁸ Recommender systems, one of the first deployed uses of machine learning, also induced demand for explanations of their specific recommendations; the most satisfying answers combined justifications based on the user's previous choices, ratings of similar users, and features of the items being recommended.³²

Locally approximate explanations. In many cases, however, even a local explanation can be too complex to understand without approximation. Here, the key challenge is deciding which details to omit when creating the simpler explanatory model. Human preferences, discovered by psychologists and summarized previously, should guide algorithms that construct these simplifications.

Ribeiro et al.'s LIME system³³ is a good example of a system for generating a locally approximate explanatory model of an arbitrary learned model, but it sidesteps part of the question of which details to omit. Instead, LIME requires the developer to provide two additional inputs: A set of semantically meaningful features X' that can be computed from the original features, and an interpretable learning algorithm, such as a linear classifier (or a GA²M), which it uses to generate an explanation in terms of the X'.

The insight behind LIME is shown in Figure 5. Given an instance to explain, shown as the bolded red cross, LIME randomly generates a set of similar instances and uses the blackbox classifier, f, to predict their values (shown as the red crosses and blue circles). These predictions are weighted by their similarity to the input instance (akin to locally weighted regression) and used to train a new, simpler intelligible classifier, shown on the figure as the linear decision boundary, using X', the smaller set of semantic features. The user receives the intelligible classifier as an explanation. While this explanation model²⁸ is likely a poor global representation of f, it is hopefully an accurate local approximation of the boundary in the vicinity of the instance being explained.

Figure 5. The intuition guiding LIME's method for constructing an approximate local explanation. Source: Ribeiro et al.³³

Ribeiro et al. tested LIME on several domains. For example, they explained the predictions of a convolutional neural network image classifier by converting the pixel-level features into a smaller set of "super-pixels;" to do so, they ran an off-the-shelf segmentation algorithm that identified regions in the input image and varied the color of some these regions when generating "similar" images. While LIME provides no formal guarantees about its explanations, studies showed that LIME's explanations helped users evaluate which of several classifiers best generalizes.

Choice of explanatory vocabulary. Ribeiro et al.'s use of presegmented image regions to explain image classification decisions illustrates the larger problem of determining an explanatory vocabulary. Clearly, it would not make sense to try to identify the exact pixel that led to the decision: pixels are too low level a representation and are not semantically meaningful to users. In fact, deep neural network's power comes from the very fact that their hidden layers are trained to recognize latent features in a manner that seems to perform much better than previous efforts to define such features independently. Deep networks are inscrutable exactly because we do not know what those hidden features denote.

To explain the behavior of such models, however, we must find some high-level abstraction over the input pixels that communicate the model's essence. Ribeiro et al.'s decision to use an off-the-shelf image-segmentation system was pragmatic. The regions it selected are easily visualized and carry some semantic value. However, regions are chosen without any regard to how the classifier makes a decision. To explain a blackbox model, where there is no possible access to the classifier's internal representation, there is likely no better option; any explanation will lack faithfulness.

However, if a user can access the classifier and tailor the explanation system to it, there are ways to choose a more meaningful vocabulary. One interesting method jointly trains a classifier with a natural language, image-captioning system.¹³ The classifier uses training data labeled with the objects appearing in the image; the captioning system is labeled with English sentences describing the appearance of the image. By training these systems jointly, the variables in the hidden layers may get aligned to semantically meaningful concepts, even as they are being trained to provide discriminative power. This results in English language descriptions of images that have both high image relevance (from the captioning training data) and high class relevance (from the object recognition training data), as shown in Figure 6.

Figure 6. A visual explanation taken from Hendricks et al.¹³

While this method works well for many examples, some explanations include details that are not actually present in the image; newer approaches, such as phrase-critic methods, may create even better descriptions.¹⁴ Another approach might determine if there are hidden layers in the learned classifier that learn concepts corresponding to something meaningful. For example, Zeiler and Fergus observed that certain layers may function as edge or pattern detectors.⁴⁰ Whenever a user can identify the presence of such layers, then it may be preferable to use them in the explanation. Bau et al. describe an automatic mechanism for matching CNN representations with semantically meaningful concepts using a large, labeled corpus of objects, parts, and texture; furthermore, using this alignment, their method quantitatively scores CNN interpretability, potentially suggesting a way to optimize for intelligible models.

However, many obstacles remain. As one example, it is not clear there are satisfying ways to describe important, discriminative features, which are often intangible, for example, textures. An intelligible explanation may need to define new terms or combine language with other modalities, like patches of an image. Another challenge is inducing first-order, relational descriptions, which would enable descriptions such as "a spider because it has eight legs" and "full because all seats are occupied." While quantified and relational abstractions are very natural for people, progress in statistical-relational learning has been slow and there are many open questions for neuro-symbolic learning.³

Facilitating user control with explanatory models. Generating an explanation by mapping an inscrutable model into a simpler, explanatory model is only half of the battle. In addition to answering counterfactuals about the original model, we would ideally be able to map any control actions the user takes in the explanatory model back as adjustments to the original, inscrutable model. For example, as we illustrated how a user could directly edit a GA²M's shape curve (Figure 4b) to change the model's response to asthma. Is there a way to interpret such an action, made to an intelligible explanatory model, as a modification to the original, inscrutable model? It seems unlikely that we will discover a general method to do this for arbitrary source models, since the abstraction mapping is not invertible in general. However, there are likely methods for mapping backward to specific classes of source models or for specific types of feature-transform mappings. This is an important area for future study.

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Toward Interactive Explanation

The optimal choice of explanation depends on the audience. Just as a human teacher would explain physics differently to students who know or do not yet know calculus, the technical sophistication and background knowledge of the recipient affects the suitability of a machine-generated explanation. Furthermore, the concerns of a house seeker whose mortgage application was denied due to a FICO score differ from those of a developer or data scientist debugging the system. Therefore, an ideal explainer should model the user's background over the course of many interactions.

The HCI community has long studied mental models,³¹ and many intelligent tutoring systems (ITSs) build explicit models of students' knowledge and misconceptions.² However, the frameworks for these models are typically hand-engineered for each subject domain, so it may be difficult to adapt ITS approaches to a system that aims to explain an arbitrary black-box learner.

Even with an accurate user model, it is likely that an explanation will not answer all of a user's concerns, because the human may have follow-up questions. We conclude that an explanation system should be interactive, supporting such questions from and actions by the user. This matches results from psychology literature, summarized earlier, and highlights Grice's maxims, especially those pertaining to quantity and relation. It also builds on Lim and Dey's work in ubiquitous computing, which investigated the kinds of questions users wished to ask about complex, context-aware applications.²⁴ We envision an interactive explanation system that supports many different follow-up and drill-down actions after presenting a user with an initial explanation:

• Redirecting the answer by changing the foil. "Sure, but why didn't you predict class C?"
• Asking for more detail (that is, a more complex explanatory model), perhaps while restricting the explanation to a subregion of feature space. "I'm only concerned about women over age 50 ..."
• Asking for a decision's rationale. "What made you believe this?" To which the system might respond by displaying the labeled training examples that were most influential in reaching that decision, for example, ones identified by influence functions¹⁹ or nearest neighbor methods.
• Query the model's sensitivity by asking what minimal perturbation to certain features would lead to a different output.
• Changing the vocabulary by adding (or removing) a feature in the explanatory model, either from a predefined set, by using methods from machine teaching, or with concept activation vectors.¹⁷
• Perturbing the input example to see the effect on both prediction and explanation. In addition to aiding understanding of the model (directly testing a counterfactual), this action enables an affected user who wants to contest the initial prediction: "But officer, one of those prior DUIs was overturned ...?"
• Adjusting the model. Based on new understanding, the user may wish to correct the model. Here, we expect to build on tools for interactive machine learning¹ and explanatory debugging,^20,21 which have explored interactions for adding new training examples, correcting erroneous labels in existing data, specifying new features, and modifying shape functions. As mentioned in the previous section, it may be challenging to map user adjustments that are made in reference to an explanatory model, back into the original, inscrutable model.

To make these ideas concrete, Figure 7 presents a possible dialog as a user tries to understand the robustness of a deep neural dog/fish classifier built atop Inception v3.³⁹ As the figure shows: (1) The computer correctly predicts the image depicts a fish. (2) The user requests an explanation, which is provided using LIME.³³ (3) The user, concerned the classifier is paying more attention to the background than to the fish itself, asks to see the training data that influenced the classifier; the nearest neighbors are computed using influence functions.¹⁹ While there are anemones in those images, it also seems that the system is recognizing a clownfish. (4) To gain confidence, the user edits the input image to remove the background, re-submits it to the classifier and checks the explanation.

Figure 7. An example of an interactive explanatory dialog for gaining insight into a DOG/FISH image classifier.

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Explaining Combinatorial Search

Most of the preceding discussion has focused on intelligible machine learning, which is just one type of artificial intelligence. However, the same issues also confront systems based on deep-lookahead search. While many planning algorithms have strong theoretical properties, such as soundness, they search over action models that include their own assumptions. Furthermore, goal specifications are likewise incomplete.²⁹ If these unspoken assumptions are incorrect, then a formally correct plan may still be disastrous.

Consider a planning algorithm that has generated a sequence of actions for a remote, mobile robot. If the plan is short with a moderate number of actions, then the problem may be inherently intelligible, and a human could easily spot a problem. However, larger search spaces could be cognitively overwhelming. In these cases, local explanations offer a simplification technique that is helpful, just as it was when explaining machine learning. The vocabulary issue is likewise crucial: how does one succinctly and abstractly summarize a complete search subtree? Depending on the choice of explanatory foil, different answers are appropriate.⁸ Sreedharan et al. describe an algorithm for generating the minimal explanation that patches a user's partial understanding of a domain.³⁷ Work on mixed-initiative planning⁷ has demonstrated the importance of supporting interactive dialog with a planning system. Since many AI systems, for example, AlphaGo,³⁵ combine deep search and machine learning, additional challenges will result from the need to explain interactions between combinatorics and learned models.

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Final Thoughts

In order to trust deployed AI systems, we must not only improve their robustness,⁵ but also develop ways to make their reasoning intelligible. Intelligibility will help us spot AI that makes mistakes due to distributional drift or incomplete representations of goals and features. Intelligibility will also facilitate control by humans in increasingly common collaborative human/AI teams. Furthermore, intelligibility will help humans learn from AI. Finally, there are legal reasons to want intelligible AI, including the European GDPR and a growing need to assign liability when AI errs.

Depending on the complexity of the models involved, two approaches to enhancing understanding may be appropriate: using an inherently interpretable model, or adopting an inscrutably complex model and generating post hoc explanations by mapping it to a simpler, explanatory model through a combination of currying and local approximation. When learning a model over a medium number of human-interpretable features, one may confidently balance performance and intelligibility with approaches like GA²Ms. However, for problems with thousands or millions of features, performance requirements likely force the adoption of inscrutable methods, such as deep neural networks or boosted decision trees. In these situations, posthoc explanations may be the only way to facilitate human understanding.

Research on explanation algorithms is developing rapidly, with work on both local (instance-specific) explanations and global approximations to the learned model. A key challenge for all these approaches is the construction of an explanation vocabulary, essentially a set of features used in the approximate explanation model. Different explanatory models may be appropriate for different choices of explanatory foil, an aspect deserving more attention from systems builders. While many intelligible models can be directly edited by a user, more research is needed to determine how best to map such actions back to modify an underlying inscrutable model. Results from psychology show that explanation is a social process, best thought of as a conversation. As a result, we advocate increased work on interactive explanation systems that support a wide range of follow-up actions. To spur rapid progress in this important field, we hope to see collaboration between researchers in multiple disciplines.

Acknowledgments. We thank E. Adar, S. Ameshi, R. Calo, R. Caruana, M. Chickering, O. Etzioni, J. Heer, E. Horvitz, T. Hwang, R. Kambhamapti, E. Kamar, S. Kaplan, B. Kim, P. Simard, Mausam, C. Meek, M. Michelson, S. Minton, B. Nushi, G. Ramos, M. Ribeiro, M. Richardson, P. Simard, J. Suh, J. Teevan, T. Wu, and the anonymous reviewers for helpful conversations and comments. This work was supported in part by the Future of Life Institute grant 2015-144577 (5388) with additional support from NSF grant IIS-1420667, ONR grant N00014-15-1-2774, and the WRF/Cable Professorship.

Figure. Watch the authors discuss this work in the exclusive Communications video. https://cacm.acm.org/videos/the-challenge-of-crafting-intelligible-intelligence

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