NBDT:
N EURAL
-B ACKED D ECISION T REE
Anonymous authors
Paper under double-blind review
A BSTRACT
Machine learning applications such as ﬁnance and medicine demand accurate and justiﬁable predictions, barring most deep learning methods from use.
In response, previous work combines decision trees with deep learning, yielding models that (1) sacriﬁce interpretability for accuracy or (2) sacriﬁce accuracy for interpretability.
We forgo this dilemma by jointly improving accuracy and interpretability using Neural-Backed Decision Trees (NBDTs).
NBDTs replace a neural network’s ﬁnal linear layer with a differentiable sequence of decisions and a surrogate loss.
This forces the model to learn high-level concepts and lessens reliance on highlyuncertain decisions, yielding (1) accuracy: NBDTs match or outperform modern neural networks on CIFAR, ImageNet and better generalize to unseen classes by up to 16%.
Furthermore, our surrogate loss improves the original model’s accuracy by up to 2%.
NBDTs also afford (2) interpretability: improving human trust by clearly identifying model mistakes and assisting in dataset debugging.
I NTRODUCTION
Many computer vision applications (e.g. medical imaging and autonomous driving) require insight into the model’s decision process, complicating applications of deep learning which are traditionally black box.
Recent efforts in explainable computer vision attempt to address this need and can be grouped into one of two categories: (1) saliency maps and (2) sequential decision processes.
Saliency maps retroactively explain model predictions by identifying which pixels most affected the prediction.
However, by focusing on the input, saliency maps fail to capture the model’s decision making process.
For example, saliency offers no insight for a misclassiﬁcation when the model is “looking” at the right object for the wrong reasons.
Alternatively, we can gain insight into the model’s decision process by breaking up predictions into a sequence of smaller semantically meaningful decisions as in rule-based models like decision trees.
However, existing efforts to fuse deep learning and decision trees suffer from (1) signiﬁcant accuracy loss, relative to contemporary models (e.g., residual networks), (2) reduced interpretability due to accuracy optimizations (e.g., impure leaves and ensembles), and (3) tree structures that offer limited insight into the model’s credibility.
To address these, we propose Neural-Backed Decision Trees (NBDTs) to jointly improve both (1) accuracy and (2) interpretability of modern neural networks, utilizing decision rules that preserve (3) properties like sequential, discrete decisions; pure leaves; and non-ensembled predictions.
These properties in unison enable unique insights, as we show.
We acknowledge that there is no universally-accepted deﬁnition of interpretability (Lundberg et al., 2020; Doshi-Velez & Kim, 2017;
Lipton, 2016), so to show interpretability, we adopt a deﬁnition offered by Poursabzi-Sangdeh et al.
(2018): A model is interpretable if a human can validate its prediction, determining when the model has made a sizable mistake.
We picked this deﬁnition for its importance to downstream beneﬁts we can evaluate, speciﬁcally (1) model or dataset debugging and (2) improving human trust.
To accomplish this, NBDTs replace the ﬁnal linear layer of a neural network with a differentiable oblique decision tree and, unlike its predecessors ( i.e. decision trees, hierarchical classiﬁers), uses a hierarchy derived from model parameters, does not employ a hierarchical softmax, and can be created from any existing classiﬁcation neural network without architectural modiﬁcations.
These improvements tailor the hierarchy to the network rather than overﬁt to the feature space, lessens the decision tree’s reliance on highly uncertain decisions, and encourages accurate recognition of high-level concepts.
These beneﬁts culminate in joint improvement of accuracy and interpretability.
Our contributions are as follows:
We propose a tree supervision loss , yielding NBDTs that match/outperform and outgeneralize modern neural networks (WideResNet, EfﬁcientNet) on ImageNet, TinyImageNet200, and CIFAR100.
Our loss also improves the original model by up to 2%.
We propose alternative hierarchies for oblique decision trees – induced hierarchies built using pre-trained neural network weights – that outperform both data-based hierarchies (e
g. built with information gain) and existing hierarchies (e.g. WordNet), in accuracy.
We show NBDT explanations are more helpful to the user when identifying model mistakes, preferred when using the model to assist in challenging classiﬁcation tasks and can be used to identify ambiguous ImageNet labels
R ELATED W ORKS
Saliency Maps.
Numerous efforts (Springenberg et al., 2014; Zeiler & Fergus, 2014; Simonyan et al., 2013; Zhang et al., 2016; Selvaraju et al., 2017; Ribeiro et al., 2016; Petsiuk et al., 2018;
Sundararajan et al., 2017) have explored the design of saliency maps identifying pixels that most inﬂuenced the model’s prediction.
White-box techniques (Springenberg et al., 2014; Zeiler & Fergus, 2014; Simonyan et al., 2013; Selvaraju et al., 2017; Sundararajan et al., 2017) use the network’s parameters to determine salient image regions, and black-box techniques (Ribeiro et al., 2016; Petsiuk et al., 2018) determine pixel importance by measuring the prediction’s response to perturbed inputs.
However, saliency does not explain the model’s decision process (e.g. Was the model confused early on, distinguishing between Animal and Vehicle ?
Or is it only confused between dog breeds?).
Transfer to Explainable Models.
Prior to the recent success of deep learning, decision trees were state-of-the-art on a wide variety of learning tasks and the gold standard for interpretability.
Despite this recency, study at the intersection of neural network and decision tree dates back three decades, where neural networks were seeded with decision tree weights (Banerjee, 1990; 1994; Ivanova &
Kubat, 1995a;b), and decision trees were created from neural network queries (Krishnan et al., 1999;
Boz, 2000; Dancey et al., 2004; Craven & Shavlik, 1996; 1994), like distillation (Hinton et al., 2015).
The modern analog of both sets of work (Humbird et al., 2018; Siu, 2019; Frosst & Hinton, 2017) evaluate on feature-sparse, sample-sparse regimes such as the UCI datasets (Dua & Graff, 2017) or
MNIST (LeCun et al., 2010) and perform poorly on standard image classiﬁcation tasks.
Hybrid Models.
Recent work produces hybrid decision tree and neural network models to scale up to datasets like CIFAR10 (Krizhevsky, 2009), CIFAR100 (Krizhevsky, 2009), TinyImageNet (Le & Yang, 2015), and ImageNet (Deng et al., 2009).
One category of models organizes the neural network into a hierarchy, dynamically selecting branches to run inference (Veit & Belongie, 2018;
McGill & Perona, 2017; Teja Mullapudi et al., 2018; Redmon & Farhadi, 2017; Murdock et al., 2016).
However, these models use impure leaves resulting in uninterpretatble, stochastic paths.
Other approaches fuse deep learning into each decision tree node: an entire neural network (Murthy et al., 2016), several layers (Murdock et al., 2016; Roy & Todorovic, 2016), a linear layer (Ahmed et al., 2016), or some other parameterization of neural network output (Kontschieder et al., 2015).
These models see reduced interpretability by using k-way decisions with large k (via depth-2 trees)
(Ahmed et al., 2016; Guo et al., 2018) or employing an ensemble (Kontschieder et al., 2015; Ahmed et al., 2016), which is often referred to as a “black box” (Carvalho et al., 2019; Rudin, 2018).
Hierarchical Classiﬁcation (Silla & Freitas, 2011).
One set of approaches directly uses a preexisting hierarchy over classes, such as WordNet (Redmon & Farhadi, 2017; Brust & Denzler, 2019; Deng et al.).
However conceptual similarity is not indicative of visual similarity .
Other models build a hierarchy using the training set directly, via a classic data-dependent metric like Gini impurity (Alaniz & Akata, 2019) or information gain (Rota Bulo & Kontschieder, 2014; Bic¸ici et al., 2018).
These models are instead prone to overﬁtting , per (Tanno et al., 2019).
Finally, several works introduce hierarchical surrogate losses (Wu et al., 2017; Deng et al., 2012), such as hierarchical softmax (Mohammed & Umaashankar, 2018), but as the authors note, these methods quickly suffer from major accuracy loss with more classes or higher-resolution images (e.g. beyond CIFAR10).
We demonstrate hierarchical classiﬁers attain higher accuracy without a hierarchical softmax. [Figure]
M ETHOD
Neural-Backed Decision Trees (NBDTs) replace a network’s ﬁnal linear layer with a decision tree.
Unlike classical decision trees or many hierarchical classiﬁers, NBDTs use path probabilities for inference (Sec 3.1) to tolerate highly-uncertain intermediate decisions, build a hierarchy from pretrained model weights (Sec 3.2 & 3.3) to lessen overﬁtting, and train with a hierarchical loss (Sec 3.4) to signiﬁcantly better learn high-level decisions (e.g., Animal vs. Vehicle ).
3. I NFERENCE
Our NBDT
ﬁrst featurizes each sample using the neural network backbone; the backbone consists of all neural network layers before the ﬁnal linear layer.
Second, we run the ﬁnal fully-connected layer as an oblique decision tree.
However, (a) a classic decision tree cannot recover from a mistake early in the hierarchy and (b) just running a classic decision tree on neural features drops accuracy signiﬁcantly, by up to 11% (Table 2).
Thus, we present modiﬁed decision rules (Figure 1, B):
Seed oblique decision rule weights with neural network weights.
An oblique decision tree supports only binary decisions, using a hyperplane for each decision.
Instead we associate a weight vector n i with each node. For leaf nodes, where
i = k 1 , K ] , each n i = w k is a row vector from the fully-connected layer’s weights W 2 R D ⇥ K .
For all inner nodes, where i 2 [ K + 1 , N ] , ﬁnd all leaves k 2 L ( i ) in node i ’s subtree and average their weights: n i =
[Equation]
Compute node probabilities.
Child probabilities are given by softmax inner products.
For each sample x and node i , compute the probability of each child j 2 C ( i ) using p ( j | i )
=
S OFTMAX ( h ~ n i , x i )[ j ] , where ~ n i = ( h n j , x i )
j 2 C ( i ) .
Pick a leaf using path probabilities.
Inspired by Deng et al.
(2012), consider a leaf, its class k and its path from the root P k .
The probability of each node i 2 P k traversing the next node in the path C k
( i ) 2 P k \ C ( i ) is denoted p ( C k ( i ) | i ) .
Then, the probability of leaf and its class k is
[Equation]
In soft inference, the ﬁnal class prediction ˆ k is deﬁned over these class probabilities,
[Equation]
Our inference strategy has two beneﬁts: (a) Since the architecture is unchanged, the fully-connected layer can be run regularly (Table 5) or as decision rules (Table 1), and (b) unlike decision trees and xxx d ... xŷŷŷŷ k www 4 wwwwwwwwwwwwww [Figure] other conditionally-executed models (Tanno et al., 2019; Veit & Belongie, 2018), our method can recover from a mistake early in the hierarchy with sufﬁcient uncertainty in the incorrect path (Figure 1 C, Appendix Table 7).
This inference mode bests classic tree inference (Appendix B.2).
3. B UILDING I NDUCED H IERARCHIES
Existing decision-tree-based methods use (a) hierarchies built with data-dependent heuristics like information gain or (b) existing hierarchies like WordNet.
However, the former overﬁts to the data, and the latter focuses on conceptual rather than visual similarity: For example, by virtue of being an animal, Bird is closer to Cat than to Plane , according to WordNet.
However, the opposite is true for visual similarity: by virtue of being in the sky, Bird is more visually similar to Plane than to Cat .
Thus, to prevent overﬁtting and reﬂect visual similarity, we build a hierarchy using model weights. Our hierarchy requires pre-trained model weights.
Take row vectors w k : k 2 [1 , K ] , each representing a class, from the fully-connected layer weights W .
Then, run hierarchical agglomerative clustering on the normalized class representatives w k / k w k k 2 .
Agglomerative clustering decides which nodes and groups of nodes are iteratively paired. As described in Sec 3.1, each leaf node’s weight is a row vector w k 2 W (Figure 2, Step B) and each inner node’s weight n i is the average of its leaf node’s weights (Figure 2, Step C).
This hierarchy is the induced hierarchy (Figure 2). 3. L ABELING D ECISION N ODES WITH W ORD N ET WordNet is a hierarchy of nouns.
To assign WordNet meaning to nodes, we compute the earliest common ancestor for all leaves in a subtree: For example, say Dog and Cat are two leaves that share a parent. To ﬁnd WordNet
meaning for the parent, ﬁnd all ancestor concepts that Dog and Cat share, like Mammal , Animal , and Living Thing .
The earliest shared ancestor is Mammal , so we assign Mammal to the parent of Dog and Cat . We repeat for all inner nodes. However, the WordNet corpus is lacking in concepts that are not themselves objects, like object attributes (e.g., Pencil and Wire are both cylindrical) and (b) abstract visual ideas like context (e.g., ﬁsh and boat are both aquatic).
Many of these which are littered across our induced hierarchies (Appendix Figure 14).
Despite this limitation, we use WordNet to assign meaning to intermediate decision nodes, with more sophisticated methods left to future work.
3. F INE - TUNING WITH T REE S UPERVISION L OSS
Even though standard cross entropy loss separates representatives for each leaf, it is not trained to separate representatives for each inner node (Table 3, “None”). To amend this, we add a tree super- [Table] NBDT
[Table]
[Equation] [Table] [Equation] vision loss , a cross entropy loss over the class distribution of path probabilities D nbdt =
{+{ p ( k ) } Kk =
+}
(Eq. 1) from Sec 3.1, with time-varying weights !
t ,  t where t is the epoch count:
[Equation]
Our tree supervision loss L soft requires a pre-deﬁned hierarchy. We ﬁnd that (a) tree supervision loss damages learning speed early in training, when leaf weights are nonsensical.
Thus, our tree supervision weight !
t grows linearly from {+! 
 =+} 0 to !
T = 0 .
5 for CIFAR10, CIFAR100, and to !
T = 5 for TinyImageNet, ImageNet;  t 2 [0 , 1] decays linearly over time.
(b) We re-train where possible, ﬁne-tuning with L soft only when the original model accuracy is not reproducible.
(c) Unlike hierarchical softmax, our path-probability cross entropy loss L soft disproportionately upweights decisions earlier in the hierarchy, encouraging accurate high-level decisions; this is reﬂected our out-generalization of the baseline neural network by up to 16% to unseen classes (Table 6).
E XPERIMENTS
NBDTs obtain state-of-the-art results for interpretable models and match or outperform modern neural networks on image classiﬁcation.
We report results on different models (ResNet, WideResNet, EfﬁcientNet) and datasets (CIFAR10, CIFAR100, TinyImageNet, ImageNet).
We additionally conduct ablation studies to verify the hierarchy and loss designs, ﬁnd that our training procedure improves the original neural network’s accuracy by up to 2%, and show that NBDTs improve generalization to unseen classes by up to 16%.
All reported improvements are absolute.
4. R ESULTS
Small-scale Datasets.
Our method (Table 1) matches or outperforms recently state-of-the-art neural networks.
On CIFAR10 and TinyImageNet, NBDT accuracy falls within 0.15% of the baseline neural network.
On CIFAR100, NBDT accuracy outperforms the baseline by ⇠ 1%.
Large-scale Dataset.
On ImageNet (Table 3), NBDTs obtain 76.60% top-1 accuracy, outperforming the strongest competitor NofE by 15%.
Note that we take the best competing results for any decision-tree-based method, but the strongest competitors hinder interpretability by using ensembles of models like a decision forest (DNDF, DCDJ) or feature shallow trees with only depth 2 (NofE). [Table] [Table] [Table]
4. A NALYSIS
Analyses show that our NBDT improvements are dominated by signiﬁcantly improved ability to distinguish higher-level concepts (e.g., Animal vs. Vehicle ).
Comparison of Hierarchies.
Table 2 shows that our induced hierarchies outperform alternatives.
In particular, data-dependent hierarchies overﬁt, and the existing WordNet hierarchy focuses on conceptual rather than visual similarity.
Comparisons of Losses.
Previous work suggests hierarchical softmax (Appendix B.1) is necessary for hierarchical classiﬁers.
However, our results suggest otherwise: NBDTs trained with hierarchical softmax see ⇠ 3% less accuracy than with tree supervision loss on TinyImageNet (Table 3).
Original Neural Network.
Per Sec 3.1, we can run the original neural network’s fully-connected layer normally, after training with tree supervision loss.
Using this, we ﬁnd that the original neural network’s accuracy improves by up to 2% on CIFAR100, TinyImageNet
(Table 5).
Zero-Shot Superclass Generalization.
We deﬁne a “superclass” to be the hypernym of several classes.
(e.g. Animal is a superclass of Cat and Dog ).
Using WordNet (per Sec 3.2), we (1) identify which superclasses each NBDT inner node is deciding between (e.g. Animal vs. Vehicle ).
We ﬁnd unseen classes that belong to the same superclass, from a different dataset.
(e.g. Pull Turtle images from ImageNet).
(3) Evaluate the model to ensure the unseen class is classiﬁed into the correct superclass (e.g. ensure Turtle is classiﬁed as Animal ).
For an NBDT, this is straightforward: one of the inner nodes classiﬁes Animal vs. Vehicle
(Sec 3.3).
For a standard neural network, we consider the superclass that the ﬁnal prediction belongs to.
( i.e. When evaluating Animal vs. Vehicle on a Turtle image, the CIFAR-trained model may predict any CIFAR Animal class).
See Appendix
A.2 for details.
Our NBDT consistently bests the original neural network by 8%+ (Table 6).
When discerning Carnivore vs. Ungulate , NBDT outperforms the original neural network by 16%.
Mid-Training Hierarchy : We test NBDTs without using pre-trained weights, instead constructing hierarchies during training from the partially-trained network’s weights.
Tree supervision loss with mid-training hierarchies reliably improve the original neural network’s accuracy, up to ⇠ 0.6%, and the NBDT itself can match the original neural network’s accuracy (Table 4).
However, this underperforms NBDT (Table 1), showing fully-trained weights are still preferred for hierarchy construction.
I NTERPRETABILITY
By breaking complex decisions into smaller intermediate decisions, decision trees provide insight into the decision process.
However, when the intermediate decisions are themselves neural network
[Table] [Table]
predictions, extracting insight becomes more challenging. To address this, we adopt benchmarks and an interpretability deﬁnition offered by Poursabzi-Sangdeh et al.
A model is interpretable if a human can validate its prediction, determining when the model has made a sizable mistake. To assess this, we adapt Poursabzi-Sangdeh et al.
(2018)’s benchmarks to computer vision and show (a) humans can identify misclassiﬁcations with NBDT explanations more accurately than with saliency explanations (Sec 5.1), (b) a way to utilize NBDT’s entropy to identify ambiguous labels (Sec.
5.4), and (c) that humans prefer to agree with NBDT predictions when given a challenging image classiﬁcation task (Sec. 5.2 & 5.3).
Note that these analyses depend on three model properties that NBDT preserves: (1) discrete, sequential decisions, so that one path is selected; (2) pure leaves, so that one path picks one class; and (3) non-ensembled predictions, so that path to prediction attribution is discrete. In all surveys, we use CIFAR10-trained models with ResNet18 backbones.
5. S URVEY : I DENTIFYING F AULTY M ODEL P REDICTIONS
In this section we aim to answer a question posed in (Poursabzi-Sangdeh et al., 2018) ” How well can someone detect when the model has made a sizable mistake?” .
In this survey, each user is given 3 images, 2 of which are correctly classiﬁed and 1 is mis-classiﬁed. Users must predict which image was incorrectly classiﬁed given a) the model explanations and b) without the ﬁnal prediction. For saliency maps, this is a near-impossible task as saliency usually highlights the main object in the image, regardless of wrong or right.
However, hierarchical methods provide a sensible sequence of We [Table] Our loss increases the original network accu- racy consistently by ⇠ .
[Table] NN- [Table] the
[Table] [Table] [Table] [Table]
intermediate decisions that can be checked. This is reﬂected in the results: For each explainability technique, we collected 600 survey responses. When given saliency maps and class probabilities, only 87 predictions were correctly identiﬁed as wrong. In comparison, when given the NBDT series of predicted classes and child probabilities (e.g., “Animal (90%) !
Mammal (95%)”, without the ﬁnal leaf prediction) 237 images were correctly identiﬁed as wrong.
Thus, respondents can better recognize mistakes in NBDT explanations nearly 3 times better.
Although NBDT provides more information than saliency maps about misclassiﬁcation, a majority – the remaining 363 NBDT predictions – were not correctly identiﬁed.
To explain this, we note that ⇠ 37% of all NBDT errors occur at the ﬁnal binary decision, between two leaves; since we provide all decisions except the ﬁnal one, these leaf errors would be impossible to distinguish.
5. S URVEY : E XPLANATION -G UIDED I MAGE C LASSIFICATION
In this section we aim to answer a question posed in (Poursabzi-Sangdeh et al., 2018) “To what extent do people follow a model’s predictions when it is beneﬁcial to do so?” . In this ﬁrst survey, each user is asked to classify a severely blurred image (Fig 4).
This survey afﬁrms the problem’s difﬁculty, decimating human performance to not much more than guessing: 163 of 600 responses are correct (27.2% accuracy). In the next survey, we offer the blurred image and two sets of predictions: (1) the original neural network’s predicted class and its saliency map, and (2) the NBDT predicted class and the sequence of decisions that led up to it (“Animal, Mammal, Cat”). For all examples, the two models predict different classes.
In 30% of the examples, NBDT is right and the original model is wrong. In another 30%, the opposite is true. In the last 40%, both models are wrong. As shown in Fig.
4, the image is extremely blurry, so the user must rely on the models to inform their prediction. When offered model predictions, in this survey, 255 of 600 responses are correct (42.5% accuracy), a 15.3 point improvement over no model guidance. We observe that humans trust NBDT-explained prediction more often than the saliency-explained predictions. Out of 600 responses, 312 responses agreed with the NBDT’s prediction, 167 responses agreed with the base model’s prediction, and 119 responses disagreed with both model’s predictions.
Note that a majority of user decisions ( ⇠ 80% ) agreed with either model prediction, even though neither model prediction was correct in 40% of examples, showing our images were sufﬁciently blurred to force reliance on the models. Furthermore, 52% of responses agreed with NBDT (against saliency’s 28%), even though only 30% of NBDT predictions were correct, showing improvement in model trust. 5. S URVEY : H UMAN -D IAGNOSED L EVEL OF T RUST The explanation of an NBDT prediction is the visualization of the path traversed.
We then compare these NBDT explanations to other explainability methods in human studies. Speciﬁcally, we ask participants to pick an expert to trust (Appendix, Figure 13), based on the expert’s explanation – a saliency map (ResNet18, GradCAM), a decision tree (NBDT), or neither. We only use samples where ResNet18 and NBDT predictions agree. Of 374 respondents that picked one method over the other, 65.9% prefer NBDT explanations; for misclassiﬁed samples, 73.5% prefer NBDT.
This supports the previous survey’s results, showing humans trust NBDTs more than current saliency techniques when explicitly asked.
5. A NALYSIS : I DENTIFYING F AULTY D ATASET L ABELS
There are several types of ambiguous labels (Figure 5), any of which could hurt model performance for an image classiﬁcation dataset like ImageNet. To ﬁnd these images, we use entropy in NBDT [Figure] E X P L A
N A T
I O N [Figure] decisions, which we ﬁnd is a much stronger indicator of ambiguity than entropy in the original neural network prediction.
The intuition is as follows: If all intermediate decisions have high certainty except for a few decisions, those decisions are deciding between multiple equally plausible cases.
Using this intuition, we can identify ambiguous labels by ﬁnding samples with high “path entropy” – or highly disparate entropies for intermediate decisions on the NBDT prediction path.
Per Figure 6, the highest “path entropy” samples in ImageNet contain multiple objects, where each object could plausibly be used for the image class.
In contrast, samples that induce the highest entropy in the baseline neural network do not suggest ambiguous labels.
This suggests NBDT entropy is more informative compared to that of a standard neural network.
C ONCLUSION
In this work, we propose Neural-Backed Decision Trees that see (1) improved accuracy: NBDTs out-generalize (16%+), improve (2%+), and match (0.15%) or outperform (1%+) state-of-the-art neural networks on CIFAR10, CIFAR100, TinyImageNet, and ImageNet.
We also show (2) improved interpretability by drawing unique insights from our hierarchy, conﬁrming that humans trust NBDT’s over saliency and illustrate how path entropy can be used to identify ambiguous labels.
This challenges the conventional supposition of a dichotomy between accuracy and interpretability, paving the way for jointly accurate and interpretable models in real-world deployments.