Title: ALL NEURAL NETWORKS ARE CREATED EQUAL

Abstract: One of the unresolved questions in deep learning is the nature of the solutions that are being discovered. We investigate the collection of solutions reached by the same network architecture, with different random initialization of weights and random mini-batches. These solutions are shown to be rather similar -more often than not, each train and test example is either classified correctly by all the networks, or by none at all. Surprisingly, all the network instances seem to share the same learning dynamics, whereby initially the same train and test examples are correctly recognized by the learned model, followed by other examples which are learned in roughly the same order. When extending the investigation to heterogeneous collections of neural network architectures, once again examples are seen to be learned in the same order irrespective of architecture, although the more powerful architecture may continue to learn and thus achieve higher accuracy. This pattern of results remains true even when the composition of classes in the test set is unrelated to the train set, for example, when using out of sample natural images or even artificial images. To show the robustness of these phenomena we provide an extensive summary of our empirical study, which includes hundreds of graphs describing tens of thousands of networks with varying NN architectures, hyper-parameters and domains. We also discuss cases where this pattern of similarity breaks down, which show that the reported similarity is not an artifact of optimization by gradient descent. Rather, the observed pattern of similarity is characteristic of learning complex problems with big networks. Finally, we show that this pattern of similarity seems to be strongly correlated with effective generalization.

1. INTRODUCTION:

The recent success of deep networks in solving a variety of classification problems effectively, in some cases reaching human-level precision, is not well understood. One baffling result is the incredible robustness of the learned models: using variants of Stochastic Gradient Descent (SGD), with random weight initialization and random sampling of mini-batches, different solutions are obtained. While these solutions typically correspond to different parameter values and possibly different local minima of the loss function, nevertheless they demonstrate similar performance reliably.

To advance our understating of this issue, we are required to compare different network instances. Most comparison approaches (briefly reviewed in Appendix A) are based on deciphering the internal representations of the learned models (see Lenc & Vedaldi, 2015; Alain & Bengio, 2016; Li et al., 2016; Raghu et al., 2017; Wang et al., 2018) . We propose a simpler and more direct approachcomparing networks by their classifications of the data. To this end, we represent each network instance by 2 binary vectors which capture the train and test classification accuracy. Each vector's dimension corresponds to the size of the train/test dataset; each element is assigned 1 if the network classifies the corresponding data point correctly, and 0 otherwise.

Recall the aforementioned empirical observation -different neural network instances, obtained by repeatedly training the same architecture with SGD while randomly sampling its initial weights, achieve similar accuracy. At the very least, this observation predicts that the test-based vector representation of different networks should have similar L 1 /L 2 norms. But there is more: it has been recently shown that features of deep networks capture perceptual similarity reliably and consistently, similarly across different instances and different architectures (Zhang et al., 2018) . These results seem to suggest that our proposed representation vectors may not only have a similar norm, but should also be quite similar as individual vectors. But similar in what way?

In this paper, we analyze collections of deep neural networks classifiers, where the only constraint is that the instances are trained on the same classification problem, and investigate the similarity between them. Using the representation discussed above, we measure this similarity by two scores, consistency score and consensus score, as defined in §2. Like other comparison approaches (see Appendix A), our analysis reveals a high level of similarity between trained networks. Interestingly, it reveals a stronger sense of similarity than previously appreciated: not only is the accuracy of all the networks in the collection similar, but so is the pattern of classification. Specifically, at each time point during the learning process (or in each epoch), most of the data points in both the train and test sets are either classified correctly by all the networks, or by none at all.

As shown in §3, these results are independent of choices such as optimization method, hyperparameter values, the detailed architecture, or the particular dataset. They can be replicated for a fixed test set even when each instance in the collection sees a different train set, as long as the training data is sampled from the same distribution. Moreover, the same pattern of similarity is observed for a wide range of test data, including out-of-sample images of new classes, randomly generated images, or even artificial images generated by StyleGAN (Karras et al., 2019) . These results are also reproduce-able across domains, and were reproduced using BiLSTM (Hochreiter & Schmidhuber, 1997) with attention (Bahdanau et al., 2014) for text classification. We may therefore conclude that different network instances compute similar classification functions, even when being trained with different training samples.

It is in the dynamic of learning, where the results of our analysis seem to go significantly beyond what has been shown before, revealing an even more intriguing pattern of similarity between trained NN instances. Since deep NNs are almost always trained using gradient descent, each network can be represented by a time series of train-based and test-based representation vectors, one per epoch. We find that network instances in the collection do not only show the same pattern of classification at the end of the training, but they also evolve in the same way across time and epochs, gradually learning to correctly or incorrectly classify the same examples in the same order. As shown in §4, these results hold regardless of the network' architecture. To drive this point home we compare a variety of public domain architectures such as VGG19 (Simonyan & Zisserman, 2014), AlexNet (Krizhevsky et al., 2012) , DenseNet (Huang et al., 2017) and ResNet-50 (He et al., 2016) . In all cases, different architectures may learn at a different pace and achieve different generalization accuracy, but they still learn in the same order. Thus all networks start by learning roughly the same examples, but the more powerful networks may continue to learn additional examples as learning proceeds. A related phenomenon is observed when extending the analysis to simpler learning paradigms, such as deep linear networks, SVM, and KNN classifiers.

Our empirical study extends to cases where these robust patterns of similarity break down, see §5. For example, when randomly shuffling the labels in a known benchmark (Zhang et al., 2016) , the agreement between different classifiers disappear. This stands in agreement with (Morcos et al., 2018) , where it is shown that networks that generalize are more similar than those that memorize.

Nevertheless, the similarity in learning dynamic is not an artifact of learnability, or the fact that the networks have converged to solutions with similar accuracy. To see this we constructed a test case where shallow CNNs are trained to discriminate an artificial dataset of images of Gabor patches (see Appendix C). Here it is no longer true that different network instances learn in the same order; rather, each network instance follows its own path while converging to the final model. The similarity in learning dynamic is likewise not an artifact of using gradient descent. To see this we use SGD to train linear classifiers to discriminate vectors sampled from two largely overlapping Gaussian distributions. Once again, each classifier follows its own path while converging to the same optimal solution.

2. METHODOLOGY AND NOTATIONS:

Given some neural network architecture f , and a labeled dataset X = {(x i , y i )} M i=1 where x i ∈ R d denotes a single data point and y i ∈ [K] its corresponding label, we define and analyze the consistency of f when repeatedly trained on X from scratch. Let S E denote the set of different extents (total epochs of X) used to train f , where |S E | = E. ∀e ∈ S E we create a collection of N instances of f , denoted F e = {f e 1 , ..., f e N }. Each instance f e i is initialized independently using Xavier initialization (Glorot & Bengio, 2010) , then trained with SGD on randomly sampled mini-batches for e epochs.

We measure the consistency of architecture f by comparing the predictions of the different instances in each collection F e , and analyze this consistency throughout the entire learning process as it changes with e. Thus for epoch e, we define the consistency score of an example (x, y) as follows:

c e (x, y) = 1 N N i=1 1 [f e i (x)=y]

The consistency score c e (x, y) measures the classifiers' agreement when x is correctly classified. However, it does not take into account the classifiers' agreement when it is not. We therefore define in addition the consensus score, a complementary score that measures the consensus of the classifiers -the largest number of classifiers that classify each example by the same label:

s e (x, y) = max k∈[K] 1 N N i=1 1 [f e i (x)=k]

We say that example (x i , y i ) is easier than example (x j , y j ) during epoch e, if c e (x i , y i ) ≥ c e (x j , y j ). We say that example (x i , y i ) is learned at least as fast as example (x j , y j ) if from some epoch e and onward, example (x i , y i ) is easier than example (x j , y j ). This is formally given by: ∃e ∈ S E : ∀e > e c e (x i , y i ) ≥ c e (x j , y j ).

When all the classifiers in F e are identical, the consistency score of each example is either 0 or 1, and its consensus is 1. This results in a perfect bi-modal distribution of consistency scores; we quantify bi-modality using the following measure suggested by Pearson (1894) for an RV X: kurtusis(X) -skewness 2 (X) -1; the lower this measure is, the more bi-modal X is. On the other hand, when all classifiers independently choose class labels, the distribution of both scores is expected to resemble a Gaussian1 , centered around the average accuracy of the classifiers for the consistency score, and a slightly higher value for the mean consensus score. It follows that the higher the mean consensus is, and the more bi-modal-like the distribution of consistency scores is around 0 and 1, the more similar the set of classifiers is.

Throughout this paper we empirically investigate the distribution of the two scores defined above, using different architectures and datasets. We also examine the effects of other factors, such as resampling the train data, or classifying out-of-sample test data. For the most part, we focus on CNNs trained on visual classification tasks (although the results are reproduced in other domains), and analyze the distribution of consistency scores throughout the entire learning process.

3. DIVERSITY IN A SINGLE ARCHITECTURE:

In this section, we investigate collections of classifiers obtained from a single NN architecture f .

Same training set. We start with the simplest condition, where all instances in collection F are obtained by training with the same training set X, with different initial conditions and with independently sampled mini-batches. When using datasets of natural images, during learning the consistency among all networks in F is high, see Figs. 1a,b and Fig. 2 .

Upon initialization, all networks are effectively i.i.d random variables, and therefore the distribution of consistency scores is approximately normal around random chance 1 -1 K for K labels. After a few epochs (in many cases a single epoch is enough, see Appendix D), the consistency distribution changes dramatically, transforming to a bi-modal distribution peaking around 0 and 1. This abrupt distribution change is robust, and rather striking: from a state where most of the examples are being classified correctly by 1 K of the networks, now most of the examples are being misclassified by all the networks, while a small fraction is being correctly classified by all the networks. When learning proceeds, the improvement in accuracy affects a shift of points from the peak at 0 to the peak at 1, and the distribution remains bi-modal, see Figs. 1a, b and Fig. 2 .

The data is learned in a specific order which is insensitive to the network initialization and the sampling of the mini-batches. This is true for both the train and test sets. It indicates that the networks capture similar functions in corresponding epochs: they classify correctly the same examples, and also consistently misclassify the same examples. Had the learning of the different network instances progressed independently, we would have seen a different dynamic: as illustrated in Fig. 1c , the distribution of consistency scores of independent learners remains Gaussian in all epochs, where the Gaussian's mean slowly shifts while tracking the improved accuracy. The consistency score is not affected by how similar the misclassifications are. For this purpose we have the consensus score, which measures consistency regardless of whether the label is true or false: a consensus score of 1 indicates that all networks have classified the datapoint in the same way, regardless of whether it is correct or incorrect. Fig. 3a shows the distribution of consensus scores for the cases depicted in Fig. 1 , showing that indeed all the network instances classify examples in almost the same way, even when they misclassify. Had the learning of the different network instances progressed independently, the dynamic would have been different as shown in Fig. 3b .

Similar results are seen when analyzing a classification problem which involves text, see Fig. 4 . We applied a BiLSTM with attention using Glove (Pennington et al., 2014) over 39K training and 1K test questions from stack overflow (Public domain dataset a). Labels consist of 20 mutually exclusive programming language tags assigned by users.

Robustness. The results reported above are extremely robust, seen in all the datasets and architectures that we have investigated. In addition to the results shown above on ImageNet (Deng et al., We investigated a variety of public domain architectures, including AlexNet, DenseNet and ResNet-50 for ImageNet, VGG19 and a stripped version of VGG (denoted st-VGG) for CIFAR-10 and CIFAR-100, and several different handcrafted networks for other data sets (see details in Appendix B). The results can be replicated when changing the following hyper-parameters: learning rate, optimizer, batch size, dropout, L2-regularization, width, length and depth of layers, number of layers, number and size of kernels, and activation functions. These hyper-parameters differ across the experiments detailed both in the main paper and in Appendix D. 2Different training sets. The observed pattern of similarity does not depend on each network instance being trained by the same train set, as long as each train set is sampled from the same distribution. To see this, we randomly split the train set into several partitions. Each of the N networks in the collection is trained using a random partition, after which we compute the distribution of consistency scores using the unmodified test set. Once again, all the networks trained with the same partition show a bi-modal distribution of consistency scores over their training partition. More interestingly, all networks, regardless of the partition used for training, show a bi-modal distribution of consistency scores over the test set as would have been the case had they been trained on the same train set, see Fig. 22 in Appendix E.

Out of sample test sets. Using the collection of ResNet-50 instances whose analysis is shown in Figs. 1, 3 , we further examined the consensus score3 of the collection on out of sample test sets as shown in Fig. 5 . We see that the consensus is always higher than expected from independent classifiers. Interestingly, the more natural the images are and the more similar the distributions of the train and test images are, the higher the consensus is and the further away it gets from the consensus of independent classifiers. 

4. CROSS ARCHITECTURES DIVERSITY:

We now extend the analysis of the previous section to include networks instances which are generated by different architectures. In §4.3 we discuss comparisons with other classifiers. The distribution of the cross-consistency score in Fig. 6a ,b implies that the two architectures, ResNet-50 and AlexNet, learn the data in the same order when trained on the same dataset. We wish to measure this implication directly. To this end, we measure for a given point and a given collection, how fast the point has been learned by all the instances in the collection. Note that the sooner a point is being learned by all network instances, the higher its average consistency should be when computed over all epochs. We therefore call a point's average consistency its accessibility score, and correlate this score across two collections to compare the order of learning.

We start by correlating the accessibility score of two collections generated by the same architecture. When comparing two collections of ResNet-50, the correlation is almost 1 (r = 0.99, p ≤ 10 -50 , Fig. 6c ). When comparing two collections of two different architectures: ResNet-50 and AlexNet (r = 0.87, p ≤ 10 -50 , Fig. 6d ) or ResNet-50 and DenseNet (r = 0.97, p ≤ 10 -50 , Fig. 6e ), once again the correlation is high. These results are quite surprising given how the error rates of the three architectures differ: AlexNet Top-1 error: 0.45, ResNet-50 Top-1 error: 0.24, DenseNet Top-1 error: 0.27. The results of comparing additional pairs of competitive ImageNet architectures are shown in Appendix D, Figs. 23,24. The results have been replicated for other datasets, including: VGG19 and st-VGG on CIFAR-10 and CIFAR-100 (Fig. 25 ).

4.2. LINEAR NETWORKS:

Convolutional Neural Networks where the internal operations are limited to linear operators (Oja, 1992) define an important class of CNNs, as their linearity is often exploited in the theoretical investigation of deep learning. It is natural to wonder, therefore, whether the bi-modal behavior observed in general CNNs also occurs in this case. The answer is in the affirmative.

We train 100 st-VGG networks on the small-mammals dataset (see Appendix C). By replacing all the activation layers by the identity operator, and changing the max-pooling into average-pooling, a linear CNN architecture is obtained. The performance of these linear networks is weaker (0.43 average accuracy) than the original non-linear networks (0.56 average accuracy). Still, the distribution of the consistency scores throughout the entire learning process is bi-modal (maximum Pearson: 0.055), and this bi-modality is even more pronounced than the bi-modality in the non-linear case (maximum Pearson: 0.22). The bi-modal dynamics of st-VGG can be seen in the top row of Fig. 7a , compared to the dynamics of linear st-VGG in similar epochs at the bottom row of Fig. 7a . 

4.3. CROSS ARCHITECTURES DIVERSITY -OTHER LEARNING PARADIGMS:

Up to now, we investigated a variety of neural network architectures, revealing a common learning pattern. Can we see the same commonalities with other classification methods and paradigms? First, we consider boosting based on linear classifiers as weak learners, because the training of both neural networks and AdaBoost share a dynamic aspect: in neural networks training accuracy increases with time due to the use of GD, while in AdaBoost accuracy increases over time due to the accumulation of weak learners. We find that in both dynamics, there are commonalities in the learning order of examples. Next, we consider other machine learning paradigms, including SVM, KNN classifier, perceptron, decision tree, random forest and Gaussian naïve Bayes. Interestingly, we still find a strong correlation with the order of learning as defined above, in that these classifiers tend to fit those examples which the neural networks learn first. These results are discussed in full in Appendix F.

5. WHEN CONSISTENCY DISTRIBUTION IS NO LONGER BI-MODAL:

In Section 3 we discussed the characteristic bi-modal distribution of consistency scores, illustrated in Fig. 1 , which has appeared in all the experiments presented until now, in both the train and test sets. We have already seen that this bi-modality weakens as the similarity between the distributions of train and test data is reduced (see Fig. 5 ). In this section, we investigate the circumstances under which the bi-modal distribution of consistency scores is no longer seen.

Learning to see Gabor patches. The bi-modal distribution of consistency scores through all stages of learning is not an inherent property of neural networks. We demonstrate this point using a dataset of artificial images, consisting of Gabor patches: the dataset contains 12 overlapping classes which differ from each other in orientation, parity and color (see Appendix C). 100 instances of st-VGG have been trained to classify this data. Now the distribution of consistency scores, shown in Fig. 8 , is no longer bi-modal. Rather, the distribution is approximately normal most of the time. As learning proceeds, the mean of the distribution slowly shifts towards 1, and the width of the distribution seems to expand. At convergence, the models have reached similar performance, and the bi-modal characteristics partially re-appears on the test data. Random labels. Bi-modality seems to be associated with successful generalization. To see this, we take the small-mammals dataset, and reshuffle the labels such that every image is assigned a random label (following Zhang et al., 2016) . In this case, training accuracy can reach 100% when disabling dropout (that acts as a regularizer), which indicates that the networks can memorize the data. Interestingly, the distribution of consistency scores in no longer bi-modal, with minimum Pearson score of 1.07 on train set and 1.35 on the test set during the entire learning process. Rather, the distribution in each epoch resembles a Gaussian centered around the mean accuracy of the networks, see Fully connected networks. Bi-modality is not an artifact of using gradient descent for optimization. This can be seen in the following analysis. Specifically, we consider a fully connected neural network architecture, with 2 intermediate layers, ELU and dropout. The networks are trained to discriminate points sampled from two largely overlapping Gaussian distributions in high dimension. The dynamic distribution of consistency scores is shown in Fig. 10 , and resembles the distribution of independent networks shown in Fig. 1c . While the final solutions of the networks are similar, the order in which examples are being learned when employing SGD optimization is different. 

6. SUMMARY AND DISCUSSION:

We empirically show that neural networks learn similar classification functions. More surprisingly with respect to earlier work, the learning dynamics is also similar, as they seem to learn similar functions also in intermediate stages of learning, before convergence. This is true for a variety of architectures, including different CNN architectures and LSTMs, irrespective of size and other hyper-parameters of the learning algorithms. We have verified this pattern of results using many different CNN architectures, including most of those readily available in the public domain, and many of the datasets of natural images which are in common use when evaluating deep learning.

The similarity of network instances is measured in the way they classify examples, including known (train) and new examples. Typically, the similarity over test data is as pronounced as it is over train data, as long as the train and test examples are sampled from the same distribution. We show that this similarity extends also to out of sample test data, but it seems to decrease as the gap between the distribution of the train data and the test data is increased. This pattern of similarity crosses architectural borders: while different architectures may learn at a different speed, the data is learned in the same order. Thus all architectures which reach a certain error rate seem to classify, for the most part, the same examples in the same manner. We also see that stronger architectures, which reach a lower generalization error, seem to start by first learning the examples that weaker architectures classify correctly, followed by the learning of some more difficult examples. This may suggest that the order in which data is learned is an internal property of the data.

We also discuss cases where this similarity breaks down, indicating that the observed similarity is not an artifact of using stochastic gradient descent. Rather, the observed pattern of similarity seems to characterize the learning of complex problems with big networks. Curiously, the deeper the network is and the more non-linearities it has, and even though the model has more learning parameters, the progress of learning in different network instances becomes more similar to each other. Un-intuitively, this suggests that in a sense the number of degrees of freedom in the learning process is reduced, and that there are fewer ways to learn the data. This effect seems to force different networks, as long they are deep enough, to learn the dataset in the same way. This counter-intuitive result joins other non-intuitive results, like the theoretical result that a deeper linear neural network converges faster to the global optimum than a shallow network (Arora et al., 2018) .

We also show that the observed pattern of similarity is strongly correlated with effective generalization. What does it tell us about the generalization of neural networks, a question which is considered by many to be poorly understood? Neural networks can memorize an almost limitless number of examples, it would seem. To achieve generalization, most training protocols employ some regularization mechanism which does not allow for unlimited data memorization. As a result, the network fits only the train and test examples it would normally learn first, which are, based on our analysis, also the "easier" (or more typical) examples. We hypothesize that this may explain why a regularized network discovers robust solutions, with little variability among its likely instances. (Kawaguchi et al., 2017) . The expressiveness of NNs is broad (Cybenko, 1989) , and they can learn any arbitrary complex function (Hornik et al., 1989) . This extended capacity can indeed be reached, and neural networks can memorize datasets with randomly assigned labels (Zhang et al., 2016) . Nevertheless, the dominant hypothesis today is that in natural datasets they "prefer" to learn an easier hypothesis that fits the data rather than memorize it all (Zhang et al., 2016; Arpit et al., 2017) . Our work is consistent with a hypothesis which requires fewer assumptions, see Section 6.

The direct comparison of neural representations is regarded to be a hard problem, due to a large number of parameters and the many underlying symmetries. Many non-direct approaches are available in the literature: (Li et al., 2016; Wang et al., 2018) compare subsets of similar features across multiple networks, which span similar low dimensional spaces, and show that while single neurons can vary drastically, some features are reliably learned across networks. (Raghu et al., 2017) proposed the SVCCA method, which can compare layers and networks efficiently, with an amalgamation of SVD and CCA. They showed that multiple instances of the same converged network are similar to each other and that networks converge in a bottom-up way, from earlier layers to deeper ones. Morcos et al. (2018) builds off the results of (Raghu et al., 2017) , further showing that networks which generalize are more similar than ones which memorize, and that similarity grows with the width of the network.

In various machine learning methods such as curriculum learning (Bengio et al., 2009) , self-paced learning (Kumar et al., 2010) and active learning (Schein & Ungar, 2007) , examples are presented to the learner in a specific order (Hacohen & Weinshall, 2019; Jiang et al., 2017) . Although conceptually similar, here we analyze the order in which examples are learned, while the aforementioned methods seek ways to alter it. Likewise, the design of effective initialization methods is a striving research area (Erhan et al., 2010; Glorot & Bengio, 2010; Rumelhart et al., 1988 ). Here we do not seek to improve these methods, but rather analyze the properties of a collection of network instances generated by the same initialization methodology.

B ARCHITECTURES:

In addition to the public domain architectures described in §3, we also experimented with some handcrafted networks. Such networks are simpler and faster to train, and are typically used to investigate the learning of less commonly used datasets, such as the small-mammals dataset and tiny ImageNet. Below we list all the architectures used in this paper.

st-VGG.:

A stripped version of VGG which we used in many experiments. It is a convolutional neural network, containing 8 convolutional layers with 32, 32, 64, 64, 128, 128, 256, 256 filters respectively. The first 6 layers have filters of size 3 × 3, and the last 2 layers have filters of size 2 × 2.

Every second layer there is followed by 2 × 2 max-pooling layer and a 0.25 dropout layer. After the convolutional layers, the units are flattened, and there is a fully-connected layer with 512 units followed by 0.5 dropout. When training with random labels, we removed both dropout layers to enable proper training, as suggested in Krueger et al. (2017) . The batch size we used was 100. The output layer is a fully connected layer with output units matching the number of classes in the dataset, followed by a softmax layer. We trained the network using the SGD optimizer, with cross-entropy loss. When training st-VGG, we used a learning rate of 0.05 which decayed by a factor of 1.8 every 20 epochs.

Small st-VGG. To compare st-VGG with another architecture, we created a smaller version of it: we used another convolutional neural network, containing 4 convolutional layers with 32, 32, 64, 64 filters respectively, with filters of size 3 × 3. Every second layer there is followed by 2 × 2 max-pooling and a 0.25 dropout layer. After the convolutional layers, the units are flattened, and there is a fully-connected layer with 128 units followed by 0.5 dropout. The output layer is a fully connected layer with output units matching the number of classes in the dataset, followed by a softmax layer. We trained the network using the SGD optimizer, with cross-entropy loss. We trained this network with the same learning rate and batch size as st-VGG.

MNIST architecture. When experimenting with the MNIST dataset, we used some arbitrary small architecture for simplicity, as most architectures are able to reach over 0.99 accuracy. The architecture we used had 2 convolutional layers, with 32 and 64 filters respectively of size 3 × 3.

After the convolutions, we used 2 × 2 max-pooling, followed by 0.25 dropout. Finally, we used a fully connected layer of size 128 followed by 0.5 dropout and Softmax. We used a learning rate of 1 for 12 epochs, using AdaDelta optimizer and a batch size of 100.

Fully connected architecture. When experimenting with fully connected networks, we used a 4 layers network, which simply flattened the data, followed by 2 fully connected layers with 1024 units, followed byn an output layer with softmax. We used 0.5 dropout after each fully connected layer. Since these networks converge fast, a wide range of learning rates can be used. Specifically, we used 0.04. We experimented with a wide range of numbers of fully connected layers, reaching similar results.

BiLSTM with Attention. When experimenting on textual data we used a GloVe embeddings, a layer of BiLSTM of size 300, 0.25 dropout and recurrent dropout, an attention layer, a fully connected layer of size 256 with 0.25 dropout and a last fully connected layer to extract output. The networks were optimized using Adam optimization with a learning rate of 0.005 and a batch size of 256.

C DATASETS:

Small Mammals. The small-mammals dataset used in the paper is the relevant super-class of the CIFAR-100 dataset. It contains 2500 train images divided into 5 classes equally, and 500 test images. Each image is of size 32 × 32 × 3. This dataset was chosen due to its small size, which allowed for efficient experimentation. All the results observed in this dataset were reproduced on large, public domain datasets, such as CIFAR-100, CIFAR-10, and ImageNet.

Insect. Similarly to the small mammals dataset, the relevant super-class of CIFAR-100.

Fish. Similarly to the small mammals dataset, the relevant super-class of CIFAR-100.

Cats and Dogs. The cats and dogs dataset is a subset of CIFAR-10. It uses only the 2 relevant classes, to create a binary problem. Each image is of size 32 × 32 × 3. The dataset is divided to 20000 train images (10000 per class) and 2000 test images (1000 per class).

Gabor. The Gabor dataset used in the paper, is a dataset we created which contains 12 classes of Gabor patches. Each class contains 100 images of Gabor patches which vary in size and orientation.

Classes differ from each other in 3 parameters: 1) Parity -there is a different class for odd and even Gabor patches (corresponding to the use of sine or cosine respectively). 2) RGB channel -each class depicts the Gabor patch in a single RGB channel. 3) Orientation -each class can have one of the following base orientations: 45 • , 90 • , 135 • , 180 • . The orientation of each class varies by ±30 • , making some of the classes non-separable, while some classes are. Code for creating this dataset will be published upon acceptance.

Gaussian. The Gaussian dataset used in the fully connected case, is a 2-classes dataset. One class is sampled from a multivariate Gaussian with mean 0 and Σ = I, while the other class is sampled from a multivariate Gaussian with mean 0.1 and Σ = I. Other choices for the mean and variance yield similar results. Each sampled vector was of dimension 3072, and then reshaped to 32 × 32 × 3 to resemble the shape of CIFAR images. Each class contained 2500 train images and 500 test images.

VGGFace2 subset. We created a classification task for face recognition, using a subset of 10 classes from VGGFace2. We chose the classes containing the largest number of images. We chose 600 images from each class arbitrarily to be the train set, while the remaining points (between 89 and 243) served as the test set. Each image was resized to 64 × 64 × 3, using center cropping while maintaining aspect ratio.

Stack Overflow. The data from Stack Overflow is publicly shared and used for tutorials. It contains 39K training samples and 1K test samples, each tagged with one of 20 programming languages as the language the question asks about. Each question must be regarded more as a paragraph than a sentence. Many words, terms and symbols are expected to be domain-dependent, and therefore under-represented in the embeddings. 

D ROBUSTNESS OF RESULTS:



Similar:



E ADDITIONAL RESULTS:

Induced class hierarchy. The ranking of training examples induced by the consistency scores typically induces a hierarchical structure over the different classes as well. To see this, we train 100 instances of st-VGG on the small-mammals dataset, and calculate for each image the most frequent class label assigned to it by the collection of networks. In Fig. 27 we plot the histogram of the consistency score (as in Fig. 1 ), but this time each image is assigned a color, which identifies its most frequent class label (1 of 5 colors). It can be readily seen that at the beginning of learning, only images from 2 classes reach a consistency score of 1. As learning proceeds, more class labels slowly emerge. This result suggests that classes are learned in a specific order, across all networks. Moreover, we can see a pattern in the erroneous label assignments, which suggests that the classifiers initially use fewer class labels, and only become more specific later on in the learning process. "aquatic-mammals", "insects" and "household furniture" with st-VGG (N = 100). The number of instances N is chosen according to our computational capabilities. However, in all cases, picking much smaller N suffice to yield the same qualitative results.

In addition to hyper-parameters which may differ between various architectures, we also experimented with changing the hyper-parameters of st-VGG trained on the small-mammals dataset, always observing the same qualitative result. All experiments used N = 100 instances. Specifically, we tried a large range of learning rates, learning rate decay, SGD and Adam optimizers, large range of batch sizes, dropout and L2-regularization.

Cross architectures diversity. In addition to the results in section 4, the same qualitative results were obtained for all 2 architectures we trained on the same unmodified dataset. We conducted the following experiments: ImageNet dataset: ResNet-50 vs DenseNet, AlexNet vs DenseNet. Aquatic-mammals and small-mammals super-classes of CIFAR-100: st-VGG vs small st-VGG, Tiny ImageNet: st-VGG vs small st-VGG, CIFAR-10 and CIFAR-100: VGG19 vs st-VGG. All of which yielding similar results to the ones analyzed in section 4.

F OTHER LEARNING PARADIGMS:

Boosting linear classifiers. We use AdaBoost (Hastie et al., 2009) with k weak linear classifiers, trained on the small-mammals dataset. As commonly observed, adding more classifiers (increasing k) improves the final performance. In Fig. 30a Other classifiers. We train several classifiers on the small-mammals dataset: SVM (accuracy: 0.48), KNN classifier (accuracy: 0.36), perceptron (accuracy: 0.35), decision tree (accuracy: 0.31), random forest (accuracy: 0.48) and Gaussian naïve Bayes classifier (accuracy: 0.38). All classifiers under-perform the CNN architecture described above (accuracy: 0.56). As in the case of boosting, for each method, most of the examples which are classified correctly are among the first to be learned by the CNN architecture. In other words, for all these methods, which achieve low accuracy, it is the easier examples which are being learned for the most part. This is illustrated in Fig. 31 , demonstrating the phenomenon that during the learning process, most of the examples that are being learned by other paradigms are the ones already learned by neural networks.

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For large enough N this follows immediately from the central limit theorem.

All the code is ready for distribution, and will be published upon acceptance.

Since the classes in the test sets are not present in the train, only the consensus score remains relevant.

Equivalence is determined up to a tolerance of ±1%; results are not sensitive to this value.

Figures and Tables:

When considering bigger classification problems such as the classification of ImageNet with big modern CNN architectures, a more intricate pattern of dynamics is evident: to begin with, all networks wrongly classify most of the examples, and correctly classify a minority of the examples. The learning process is revealed by examples moving from one end (100% false classification) to the other end (100% correct classification), which implies two things: (i) the networks learn to correctly classify examples in the same order; (ii) the networks agree on the examples they misclassify throughout.

Figure 1: The distribution of consistency scores throughout the entire learning process, for 27 instances of ResNet-50 trained over ImageNet: a) Train set; b) Test set. Epochs shown, from left to right: 1, 2, 5, 30, 40, 70, 100. All the networks converged before the final epoch. c) Distribution of consistency scores over ImageNet test set, using independent models with accuracy matched to b).

Figure 3: a) The distribution of consensus scores of 27 instances of ResNet-50 models trained on ImageNet, in corresponding epochs as in Fig. 1b. b) Illustration of the distribution of consensus scores using independent models with similar accuracies as a), cf. with Fig. 1c.

Figure 5: The distribution of consensus score 3 for 27 instances of ResNet-50 trained on ImageNet, which are used to classify the following test dataset: (a) Random classification. (b) Images generated by the random sampling of pixels from a normal distribution. (c) Image generated by StyleGAN trained on ImageNet. (d) Natural Images from a different dataset -Indoor Scene Recognition (Quattoni & Torralba, 2009). (e) ImageNet test set.

4.1 DIFFERENT PUBLIC DOMAIN CONVOLUTIONAL NEURAL NETWORKS We start by directly extending the previous analysis to two collections generated by two different architectures. Each architecture is characterized by its own learning pace, therefore it makes little sense to compare consistency epoch-wise. Instead, we first match epochs between the two collections: in a matched pair of epochs [e 1 , e 2 ], by definition the mean error of the first collection in epoch e 1 is equivalent to the mean error of the second collection in epoch e 2 4 . For each pair of epochs, we merge the corresponding collections of the two architectures and compute the consistency of the merged collection. We call this score cross-consistency. In Fig. 6a,b we plot the distribution of the cross-consistency score in two matched pairs of epochs, when comparing ResNet-50 and AlexNet; comparative results for additional pairs of architectures are shown in Appendix D, Figs. 23,24. As before, the dynamics of the cross-consistency is bi-modal (mean Pearson of 0.54 for test and 0.65 for train), suggesting that the network instances of both architectures learn similar classifiers.

Figure 6: Comparing collections of networks of different architectures -ResNet-50, AlexNet, and DenseNet, trained on ImageNet. (a-b) The distribution of the cross-consistency score in two matching pairs of epochs. (a) Accuracy 0.39: ResNet-50 epoch 4, Alexnet epoch 30. (b) Accuracy 0.71: ResNet-50 epoch 40, Alexnet epoch 80. (c-e) The correlation of the accessibility score in 3 pairs of collections of different architectures trained on ImageNet. X-axis and Y -axis: accessibility score for the first and second collection in the pair respectively. c) Two collections of ResNet-50, 13 instances in each. (d) 27 instances of ResNet-50 and 22 instances of AlexNet. (d) 27 instances of ResNet-50 and 6 instances of DenseNet.

Figure 7: Comparison of linear and non linear networks. (a) Distribution of consistency scores during the learning process using: Top -st-VGG, Bottom -linear st-VGG. Epochs shown, from left to right: 0, 10, 30, 90, 140. (b) Non-linear networks easily learn the same points that are easily learned by linear networks (in red), and more (in blue), see text.

Figure 8: Artificial dataset of Gabor patches, 100 st-VGG networks. The distribution of consistency scores is shown at epochs: 0, 1, 5, 10, 20, 40, 100. Top: train data; bottom: test data.

Figure 9: Small-mammals dataset, randomly shuffled labels, 100 st-VGG networks. The distribution of consistency scores is shown at epochs: 0, 1, 5, 20, 30, 40, 50. Top: train data; bottom: test data.

Figure 10: Consistency distribution of 100 fully connected networks on test data. Epochs shown: 0, 1, 2, 3, 5, 10, 20.

Figure 11: The distribution of consistency scores during the learning process of 100 instances of small architecture (see Appendix B) trained on MNIST. Epochs shown: 0, 1, 2, 3, 5, 10, 20. We used a low learning rate (0.001) to avoid convergence after one epoch. a) Train set; b) Test set.

Figure 24: The correlation of the accessibility score in 2 pairs of collections of different architectures trained on ImageNet. Left: 27 instances of ResNet-50 and 6 instances of DenseNet, r = 0.97, p < 10 -50 . Right: 22 instances of AlexNet and 6 instances of DenseNet r = 0.87, p < 10 -50 . These results follow the same protocol described in Fig. 6.

Figure 27

Figure 28: consistency score over time

Figure 30: (a) Adaboost accuracy as a function of the number of classifiers, for easy, intermediate, and hard examples as grouped by the consistency score of a CNN. (b) Correlation between the measured difficulty based on Adaboost (X-axis) and CNN (Y-axis), with r = 0.83, p ≤ 10 -10 .

Figure 31: Fraction of correctly classified examples by each classifier, among all examples that are classified correctly (blue) and incorrectly (red) by a CNN. Left: the beginning of learning (epoch 1), right: end of learning (epoch 140). The fraction is much higher among the easier examples.

Karen Simonyan and Andrew Zisserman. Very deep convolutional networks for large-scale image recognition. arXiv preprint arXiv:1409.1556, 2014. Liwei Wang, Lunjia Hu, Jiayuan Gu, Zhiqiang Hu, Yue Wu, Kun He, and John Hopcroft. Towards understanding learning representations: To what extent do different neural networks learn the same representation. In Advances in Neural Information Processing Systems, pp. 9584-9593, 2018.

Table content: <table><tr><td>Chiyuan Zhang, Samy Bengio, Moritz Hardt, Benjamin Recht, and Oriol Vinyals. Understanding</td></tr><tr><td>deep learning requires rethinking generalization. arXiv preprint arXiv:1611.03530, 2016.</td></tr><tr><td>Richard Zhang, Phillip Isola, Alexei A. Efros, Eli Shechtman, and Oliver Wang. The unreasonable</td></tr><tr><td>effectiveness of deep features as a perceptual metric. In The IEEE Conference on Computer Vision</td></tr><tr><td>and Pattern Recognition (CVPR), June 2018.</td></tr><tr><td>A RELATED WORK</td></tr><tr><td>How deep neural networks generalize is an open problem</td></tr></table>

Peer review (iclr2020):
The paper analyzes collections of deep neural network classifiers on different dataset: the only constraint is that the different models are trained on the same classification problem. Their similarity is based on the accuracy performance on both the training and test sets. The paper shows that the accuracy and pattern of classification are similar across the different models. In other words, most data points in both the training and test sets are classified correctly by all the networks, or by none at all. The paper is well-written, and the motivation is clear. However, multiple papers in the literature already exist about the importance of initialization, and the difference in generalization for different local minima obtained by neural networks. My vote is borderline reject for the following reasons: - Although the paper has some interesting insights, the whole interpretation of the model is based on the two evaluation metrics introduced in Section 2. These two evaluation metrics measure how much different models agree for the prediction of examples, and allow to measure the *easiness* of examples. The *easiness* of examples was already studied in curriculum learning. The paper could be improved by using other qualitative evaluation metrics. - The paper studies the consensus of models for different types of architecture/datasets/optimizers (all the compared models share the architecture/datasets/optimizers). However, all the models also seem to share the same type of inialization: according to Section 2, all the neural networks have the same type of initialization (Xavier initialization in the first paragraph). Does using different types of initialization also have an impact? Maybe different types of initialization wouldn*t lead to the same conclusions. - The paper seems to consider only the agreements on the training and test sets, what about the validation set? - the *examples which are easy for linear networks are for the most part also easy for non-linear networks, but not vice versa. Thus, the non-linear networks classify correctly most of the examples that are classified by the linear networks*. This result is related to curriculum learning: although curriculum learning does not explicitly consider linear models, it considers simpler models that are easier/more efficient to train. - *Curiously, the deeper the network is and the more non-linearities it has, and even though the model has more learning parameters, the progress of learning in different network instances becomes more similar to each other. Un-intuitively, this suggests that in a sense the number of degrees of freedom in the learning process is reduced, and that there are fewer ways to learn the data*. Isn*t this observation related to the choice of activation functions? For what kinds of activation functions did you observe that phenomenon? - The results about the *out of sample test sets* seem straightforward. Test examples from a different distribution are more likely to meet less consensus since the neural networks are trained for the distribution of the training set. - Before deep learning became popular, most machine learning models (e.g. SVMs) were interested in convex optimization to avoid this kind of phenomenon: even if linear problems were trained with SGD, the model that was learned was close to a global minimum of the (convex) optimization problem, and the performance of the models was robust to their initialization. In Section 4.2, a NN is trained with linear activation and average pooling, which makes the optimization problem convex if the loss is convex. This part of the paper is related to the argument of most ML research from the 2010s (that obtained a solution close to the global minimum), just like the part where SVMs are learned. - I did not understand the KNN classification part which usually does not learn parameters and should then not depend on initialization. How are the conclusions drawn for KNN classification?