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Explainable AI: A Brief Survey on History, Research Areas, Approaches and Challenges

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Deep learning has made significant contribution to the recent progress in artificial intelligence. In comparison to traditional machine learning methods such as decision trees and support vector machines, deep learning methods have achieved substantial improvement in various prediction tasks. However, deep neural networks (DNNs) are comparably weak in explaining their inference processes and final results, and they are typically treated as a black-box by both developers and users. Some people even consider DNNs (deep neural networks) in the current stage rather as alchemy, than as real science. In many real-world applications such as business decision, process optimization, medical diagnosis and investment recommendation, explainability and transparency of our AI systems become particularly essential for their users, for the people who are affected by AI decisions, and furthermore, for the researchers and developers who create the AI solutions. In recent years, the explainability and explainable AI have received increasing attention by both research community and industry. This paper first introduces the history of Explainable AI, starting from expert systems and traditional machine learning approaches to the latest progress in the context of modern deep learning, and then describes the major research areas and the state-of-art approaches in recent years. The paper ends with a discussion on the challenges and future directions.
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Explainable AI: A Brief Survey on History,
Research Areas, Approaches and Challenges
Feiyu Xu1, Hans Uszkoreit2, Yangzhou Du1, Wei Fan1, Dongyan Zhao3, and
Jun Zhu4
1AI Lab, Lenovo Research, Lenovo Group, China
{fxu,duyz1,fanwei2}@lenovo.com
2DFKI GmbH, Germany and Giance Technologies
uszkoreit@dfki.de
3Institute of Computer Science and Technology, Peking University, China
zhaody@pku.edu.cn
4Department of Computer Science and Technology, Tsinghua University, China
dcszj@mail.tsinghua.edu.cn
Abstract. Deep learning has made significant contribution to the recent
progress in artificial intelligence. In comparison to traditional machine
learning methods such as decision trees and support vector machines,
deep learning methods have achieved substantial improvement in various
prediction tasks. However, deep neural networks (DNNs) are comparably
weak in explaining their inference processes and final results, and they
are typically treated as a black-box by both developers and users. Some
people even consider DNNs (deep neural networks) in the current stage
rather as alchemy, than as real science. In many real-world applications
such as business decision, process optimization, medical diagnosis and
investment recommendation, explainability and transparency of our AI
systems become particularly essential for their users, for the people who
are affected by AI decisions, and furthermore, for the researchers and
developers who create the AI solutions. In recent years, the explainabil-
ity and explainable AI have received increasing attention by both re-
search community and industry. This paper first introduces the history
of Explainable AI, starting from expert systems and traditional machine
learning approaches to the latest progress in the context of modern deep
learning, and then describes the major research areas and the state-of-
art approaches in recent years. The paper ends with a discussion on the
challenges and future directions.
Keywords: Explainable artificial intelligence ·intelligible machine learn-
ing ·explainable interfaces ·XAI ·interpretability
1 A Brief History of Explainable AI
In wiktionary, the word “explain” means for humans “to make plain, manifest,
or intelligible; to clear of obscurity; to illustrate the meaning of” [23]. In scientific
research, a scientific explanation is supposed to cover at least two parts: 1) the
2 F. Xu et al.
object to be explained (the “Explanandum” in Latin), and 2) the content of
explanation (the “Explanans” in Latin).
Explainable AI is not a new topic. The earliest work on Explainable AI could
be found in the literature published forty years ago [16] [19], where some expert
systems explained their results via the applied rules. Since AI research began,
scientists have argued that intelligent systems should explain the AI results,
mostly when it comes to decisions. If a rule-based expert system rejects a credit
card payment, it should explain the reasons for the negative decision. Since the
rules and the knowledge in the expert systems are defined and formulated by
human experts, these rules and knowledge are easy for humans to understand and
interpret. Decision tree is a typical method designed with explainable structure.
As illustrated in Fig. 1, starting at the top and going down, the solution path
in the decision tree presents the reasoning of a final decision.
Fig. 1. An example of decision tree, used by starting at the top and going down, level
by level, according to the defined logic. (Image courtesy of J. Jordan [10])
However, Explainable AI has become a new research topic in the context of
modern deep learning. Without completely new explanatory mechanisms, the
output of today’s Deep Neural Networks (DNNs) cannot be explained, neither
by the neural network itself, nor by an external explanatory component, and
not even by the developer of the system. We know that there are different archi-
tectures of DNNs designed for different problem classes and input data, such as
CNN, RNN, LSTM, shown in Fig. 2. All of them have to be considered as black
boxes - whose internal inference processes are neither known to the observer nor
interpretable by humans [7].
Explainability of a machine learning model is usually inverse to its prediction
accuracy - the higher the prediction accuracy, the lower the model explainability.
The DARPA Explainable AI (XAI) program presents a nice chart to illustrate
this interesting phenomena, as shown in Fig. 3, where decision trees have an
excellent degree of explainability but exhibit worst prediction accuracy among
the listed learning techniques. In the other extreme, Deep Learning methods are
Title Suppressed Due to Excessive Length 3
Fig. 2. A chart of several typical Deep Neural Networks (DNNs). (Image courtesy of
Fjodor Van Veen [22])
better in predictive capacity than any other learning methods but they are least
likely to be explicable.
Fig. 3. Explainability of machine learning models appear inverse to their prediction
accuracy. (Image courtesy of DARPA [21])
In recent years, AI researchers aim to open the black-box of neural networks
and turn it into a transparent system. As shown in Fig. 4, there are two main
strands of work in Explainable AI - transparency design and post-hoc expla-
nation. The transparency design reveals how a model functions, in the view of
developers. It tries to (a) understand model structure, e.g., the construction of
a decision tree; (b) understand single components, e.g., a parameter in logistic
regression; (c) understand training algorithms, e.g., solution seeking in a con-
vex optimization. The post-hoc explanation explains why a result is inferred, in
the view of users. It tries to (d) give analytic statements, e.g. why a goods is
recommended in a shopping website; (e) give visualizations, e.g. saliency map
is used to show pixel importance in a result of object classification; (f ) give ex-
4 F. Xu et al.
planations by example, e.g. K-nearest-neighbors in historical dataset are used to
support current results. A thorough description of the categorization of expla-
nation methods is found in Lipton et al. [13]. A comprehensive survey on recent
development of Explainable AI is provided in [5].
Fig. 4. Two categories of Explainable AI work: transparency design and post-hoc ex-
planation.
2 Relevance of Explainable AI
Increasing attention has recently been paid to Explainable AI across the world
both in research and in industry. In April 2017, DARPA funded the “Explain-
able AI (XAI) program”, aimed at improving explainability of AI decision [21].
In July 2017, the Chinese government released “The Development Plan for New
Generation of Artificial Intelligence” to encourage high-explainability AI and
strong-extensibility AI [18]. In May 2018, the “General Data Protection Regula-
tion” (GDPR) was published, in which the European Union grants their citizens
a “right to explanation” if they are affected by algorithmic decision-making [6].
Explainable AI will become increasingly important to all groups of stakeholders,
including the users, the affected people, and the developers of AI systems.
Explainable AI is important to the users who utilize the AI system. When
the AI recommends a decision, the decision makers would need to understand
the underlying reason. For example, medical doctor needs to understand what
pathological features in the input data were were guiding the algorithm before
accepting auto-generated diagnosis reports. A maintenance engineer needs to
understand which abnormal phenomena were captured by the inference algo-
rithm before following the repair recommendations. A financial investor wants
to understand what influencing factors were regarded as the critical ones by the
system algorithm before making the final investment decision. We have to verify
that the AI inference works as expected, because wrong decisions can be costly
and dangerous. Caruana et al. [3] presented a famous example “Pneumonia -
Asthma” to illustrate this point. An AI system which had been trained to pre-
dict the pneumonia risk of a person arrived at totally wrong conclusions. From
real data the model had learned that asthmatic patients with heart problems
have a much lower risk of dying of pneumonia than healthy persons. This cannot
be true since asthma is a factor that negatively affects the recovery. The training
Title Suppressed Due to Excessive Length 5
data were systematically biased, because in contrast to healthy persons, the ma-
jority of these asthma patients were under strict medical supervision. Hence this
group had a significant lower risk of dying of pneumonia. It should be noted,
though, that both the learning and the inference algorithms probably worked
correctly and also that the training data represented real cases. The insight that
the selection of the training data was not appropriate for predictions affecting
other populations may remain undiscovered if we have a black-box AI system.
Explainable AI is important to the people who are affected by AI decision. If
the AI makes its own decisions, e.g., braking of the car, shutting down a plant,
selling shares, assessing a job, issuing a traffic punishment order, the affected
people must be able to understand the reason. There are already legal regula-
tions that codify this demand [6]. Houston schools were using an AI algorithm,
called Educational Value-Added Assessment System (EVAAS), to evaluate the
performance of teachers. However, this AI system was successfully contested by
teachers in court, because negative reviews of teachers could not be explained
by the AI system [2].
Explainable AI could help developers to improve AI algorithm, by detecting
data bias, discovering mistakes in the models, and remedying the weakness. La-
puschkin et al. [11] presented an impressive example. As shown in Fig. 5, they
observed that the Fisher Vector method usually shows lower accuracy than Deep
Neural Networks in the task of object recognition. However, two methods reach
almost equal accuracy of recognition rate in the category “horse”, which is unex-
pected. A saliency map method called “Layer-wise Relevance Propagation” [12]
was then employed to analyze which pixel areas exactly make the models arrive
at their predictions. The authors observed that the two models use different
strategies to classify images of that category. The Deep Neural Network looked
at the contour of the actual horse, whereas the Fisher Vector model mostly relied
on a certain copyright tag, that happens to be present on many horse images.
Removing the copyright tag in the test images would consequently significantly
decrease the accuracy of the Fisher Vector model.
Fig. 5. Upper: the prediction accuracy of Fisher Vector and Deep Neural Network in
tasks of object recognition; Lower: model diagnosis using saliency map method. (Image
courtesy of Lapuschkin et al. [11])
6 F. Xu et al.
3 Relevant Explainable AI Problems and Current
Approaches
As shown in Fig. 6, there are three typical approaches to understand the behavior
of a Deep Neural Network: (a) making the parts of the network transparent - the
color of the neuron indicates its activation status; (b) learning semantics of the
network components - a neuron could have a meaning if it is often activated by
a certain part of the object; (c) generation of explanations - a human-readable
textual explanation tells the underlying reason to support current decision.
Fig. 6. Three approaches for understanding a neural network, indicated by red-boxes
(a), (b) and (c)
3.1 Making the parts in DNN transparency
This section introduces two popular techniques, namely sensitivity analysis (SA) [15]
[1] and layer-wise relevance propagation (LRP) [17], for explaining prediction of
deep learning models.
SA explains a prediction based on the model’s locally evaluated gradient.
Ri=k
∂xi
f(x)k.(1)
It assumes that the most relevant input features are the most sensitive for the
output. SA doesn’t explain the function value f(x), but rather quantifies the
importance of each input variable xi.
In contrast to SA, LRP explains predictions relative to the state of maximum
uncertainty. It redistributes the prediction f(x) backwards using local redistri-
bution rules until it assigns a relevance score Rito each input variable. The
relevance score Riof each input variable determines the variable’s importance
Title Suppressed Due to Excessive Length 7
to the prediction.
X
i
Ri=X
j
Rj=... =X
k
Rk=... =f(x).(2)
The Relevance conservation is the key property of the redistribution process.
This property ensures that no relevance is artificially added or removed during
redistribution. Thus, LRP truly decomposes the function values f(x) in contrast
to SA.
Fig. 7. Explaining predictions of an AI system using SA and LRP. (Image courtesy of
W. Samek [15])
Fig. 7 summarizes the process of explanation. The AI system correctly clas-
sifies the input image as “rooster”. SA indicates yellow flowers which occlude
part of the rooster need to be changed to make the image look more like the
predicted. However, such result would not indicate which pixels are actually piv-
otal for the prediction “rooster”. In contrast to SA, the heatmap computed with
LRP identifies pixels which are pivotal for the prediction “rooster”.
Additionally, SA and LRP are evaluated on three different classification tasks,
namely the annotation of images, the classification of text documents and the
recognition of human actions in videos. Fig. 8(A) shows two images from the
ILSVRC2012 [4] dataset, which have been correctly classified as “volcano” and
“coffee cup”, respectively. From the figure, we can see that SA heatmaps are
much noisier than the ones computed with LRP. SA doesn’t indicate how much
every pixel contributes to the prediction. LRP produces better explanations than
SA. Fig. 8(B) shows SA and LRP heatmaps overlaid on top of a document from
the 20Newsgroup dataset. In contrast to LPR, SA methods don’t distinguish be-
tween positive and negative evidence. Similarly, Fig. 8(C) shows LRP heatmaps
8 F. Xu et al.
not only visualizes the relevant locations of the action within a video frame, but
also identifies the most relevant time points within a video sequence.
Fig. 8. Explaining prediction of three different problems using SA and LRP. (Image
courtesy of W. Samek [15])
3.2 Learning semantic graphs from existing DNNs
Zhang et al. [24] proposes a method that learns a graphical model, called “ex-
planatory graph”, which reveals the knowledge hierarchy hidden inside a pre-
trained Convolutional Neural Network (CNN), as shown in Fig. 9. The graph
consists of multiple layers, each of them corresponds to a convolutional layer
in the CNN. Each node in the graph represents a specific part of the detected
object, as shown in the right side of the figure. These nodes are derived from re-
sponses of CNN filters with a disentangle algorithm. The edge connecting nodes
indicates their co-activation relationship in filter response and the spatial rela-
tionship in parts location. The layer shows different granularity of the part of
objects - larger parts appear in higher layers while smaller parts appear in lower
layers. This work, however, adopts an explanatory graph as a bridge to under-
stand the ordinary CNN. In later work [25], the authors introduce additional
losses to force each convolutional filter in CNN to represent a specific object
part directly, and produce an interpretable CNN.
Title Suppressed Due to Excessive Length 9
Fig. 9. An explanatory graph represents the knowledge hierarchy hidden in convolu-
tional layers of a CNN. (Image courtesy of Zhang et al. [24])
3.3 Generation of Explanations
This section introduces a novel framework which provides visual explanations
of a visual classifier [8]. Visual explanations are both image relevant and class
relevant. From Fig. 10 we can find image descriptions provides a sentence based
on visual information but not necessarily class relevant, while class definitions are
class relevant but not necessarily image relevant. In contrast, Visual explanation
such as “This is a western grebe because this bird has a long white neck, pointy
yellow beak, and a red eye.” includes the “red eye” property which is important
to distinguish between “western grebe” and “laysan albatross”. Therefore, Visual
explanations are both image relevant and class relevant. It explains why the
predicted category is the most appropriate for the image.
Fig. 10. Visual explanations are both image relevant and class relevant. (Image cour-
tesy of L. A. Hendricks [9])
Fig. 11 shows the generation of explanatory text on both an image and a
predicted class label. The input is run through a deep fine-grained recognition
pipeline to pick out nuanced details of the image and classifying it. The features
10 F. Xu et al.
and the label are then forwarded to the LSTM stack to produce a sequence of
words.
Fig. 11. Generation of explanatory text with joint classification and language model.
(Image courtesy of L. A. Hendricks [9])
4 Challenges and Future Directions
The development of Explainable AI is facing both scientific and social demands.
We expect AI systems could help humans make decisions in mission-critical
tasks. Therefore, We need a more trustworthy and transparent AI, instead of
alchemy AI [20]. Ali Rahimi, the winner of the test-of-time award in NeurIPS
2017, expressed his expectations concerning AI solutions as follows: “We are
building systems that govern healthcare and mediate our civic dialogue. We
would influence elections. I would like to live in a society whose systems are built
on top of verifiable, rigorous, thorough knowledge, and not on alchemy. Let’s take
machine learning from alchemy to electricity” [14]. The term “electricity” in his
speech could be replaced by “chemistry” from our perspective, meaning that AI
Deep Learning AI should become part of science.
DARPA has invested 50 Million USD and launched a 5-year research pro-
gram on Explainable AI (XAI) [21], aiming to produce “glass-box” models that
are explainable to a “human-in-the-loop”, without greatly sacrificing AI perfor-
mance, as shown in Fig. 12. Human users should be able to understand the AI’s
cognition both in real-time and after the results achieved, and furthermore might
be able to determine when to trust the AI and when the AI should be distrusted.
In Phase 1, it is planned to achieve initial implementations of their explainable
learning systems. In Phase 2, it is to build a toolkit library consisting of machine
learning and human-computer interface software modules that could be utilized
for developing future explainable AI systems.
It is known that humans can acquire and use both explicit knowledge and
implicit knowledge. Moreover, humans can combine the two forms of knowledge
to a certain degree. For humans, understanding and explaining require explicit
knowledge. However, DNNs acquire and use implicit knowledge in the form of
probabilistic models. As they stand, they cannot understand anything. Other
AI methods model explicit knowledge, such as Knowledge Graphs. Today the
two worlds in AI technology are still largely separated. Researchers are now
Title Suppressed Due to Excessive Length 11
Fig. 12. Explainable AI (XAI) Concept presented by DARPA. (Image courtesy of
DARPA XAI Program [21])
strengthening their efforts to bring the two worlds together. The need-driven
research on Explainable AI is a source and a catalyst for the work dedicated to
this grand challenge.
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