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Title: ON BREIMAN’S DILEMMA IN NEURAL NETWORKS: SUCCESS AND FAILURE OF NORMALIZED MARGINS

Paper content:
A belief persists long in machine learning that enlargement of margins over training data accounts for the resistance of models to overfitting by increasing the robustness.
Yet Breiman shows a dilemma (Breiman, 1999) that a uniform improvement on margin distribution \emph{does not} necessarily reduces generalization error.
In this paper, we revisit Breiman's dilemma in deep neural networks with recently proposed normalized margins using Lipschitz constant bound by spectral norm products.
With both simplified theory and extensive experiments, Breiman's dilemma is shown to rely on dynamics of normalized margin distributions, that reflects the trade-off between model expression power and data complexity.
When the complexity of data is comparable to the model expression power in the sense that training and test data share similar phase transitions in normalized margin dynamics, two efficient ways are derived via classic margin-based generalization bounds to successfully predict the trend of generalization error.
On the other hand, over-expressed models that exhibit uniform improvements on training normalized margins may lose such a prediction power and fail to prevent the overfitting.
Margin, as a measurement of the robustness allowing some perturbations on classifier without changing its decision on training data, has a long history in characterizing the performance of classification algorithms in machine learning.
As early as BID17 , it played a central role in the proof on finite-stopping or convergence of perceptron algorithm when training data is separable.
Equipped with convex optimization technique, a plethora of large margin classifiers are triggered by support vector machines BID3 BID23 .
AdaBoost, an iterative algorithm to combine an ensemble of classifiers proposed by BID4 , often exhibits a resistance to overfitting phenomenon that during the training process the generalization error keeps on non-increasing when the training error drops to zero.
Toward deciphering the such a resistance of overfitting phenomenon, BID19 proposed an explanation that the training process keeps on improving a notion of classification margins in boosting, among later works on consistency of boosting with early stopping regularization BID2 BID30 BID28 .
Lately such a resistance to overfitting is again observed in deep neural networks with overparameterized models .
A renaissance of margin theory is proposed by BID0 with a normalization of network using Lipschitz constants bounded by products of operator spectral norms.
It inspires many further investigations in various settings BID14 BID16 BID12 .However
, the improvement of margin distributions does not necessarily guarantee a better generalization performance, which is at least traced back to BID1 in his effort to understanding AdaBoost. In this
work, Breiman designed an algorithm arc-gv such that the margin can be maximized via a prediction game, then he demonstrated an example that one can achieve uniformly larger margin distributions on training data than AdaBoost but suffer a higher generalization error. In the
end of this paper, Breiman made the following comments with a dilemma: "The results above leave us in a quandary. The laboratory results for various arcing algorithms are excellent, but the theory is in disarray. The evidence is that if we try too hard to make the margins larger, then overfitting sets in. ... My sense of it is that we just do not understand enough about what is going on."Breiman's dilemma triggers some further explorations to understand the limitation of margin theory in boosting BID18 Wang et al., 2008; BID27 . In particular
, BID18 points out that the trees found by arg-gv have larger model complexity in terms of deeper average depth than AdaBoost, suggesting that margin maximization in arc-gv does not necessarily control the model complexity. The latter works
provide tighter bounds based on VC-dimension and optimized quantile training margins, which however do not apply to over-parametrized models in deep neural networks and the case where the training margin distributions are uniformly improved.In this paper, we are going to revisit Breiman's dilemma in the scenario of deep neural networks. Both the success
and failure can be seen on normalized margin based bounds on generalization error. First of all, let
's look at the following illustration example.Example (Breiman's Dilemma with a CNN). A basic 5-layer
convolutional neural network of c channels (see Section 3 for details) is trained with CIFAR-10 dataset whose 10 percent labels are randomly permuted. When c = 50 with
92, 610 parameters, FIG0 shows the training error and generalization (test) error in solid curves. From the generalization
error in (a) one can see that overfitting
indeed happens after about 10 epochs, despite that training error continuously drops down to zero. One can successfully predict such
an overfitting phenomenon from FIG0 (b), the evolution of normalized
margin distributions defined later in this paper. In (b), while small margins are
monotonically
improved during training, large margins undergoes a phase transition from increase to decrease around 10 epochs such that one can predict the tendency of generalization error in (a) using large margin dynamics. Two particular
sections of large margin dynamics
are highlighted in (b), one at 8.3 on x-axis that measures the percentage
of normalized training margins no more than 8.3 (training margin error) and the other at 0.8 on y-axis that measures the normalized margins at quantile q = 0.8 (i.e. 1/γ q,t ). Both of them meet the tendency of generalization error
in (a) and find good early stopping time to avoid overfitting
. However, as we increase the channel number to c = 400 with
about 5.8M parameters and retrain the model, (c) shows a similar overfitting phenomenon in generalization
error; on the other hand, (d) exhibits a monotonic improvement of normalized margin distributions
without a phase transition during the training and thus fails to capture the overfitting. This demonstrates the Breiman's dilemma in CNN. A key insight behind this
dilemma, is that one needs a trade-off between
the model expression power and the complexity of the dataset to endorse margin bounds a prediction power. On one hand, when the model has a limited expression power relative to the
training dataset, in the sense that the training margin distributions CAN NOT be uniformly improved during training, the generalization or test error may be predicted from dynamics of normalized margin distributions. On the other hand, if we push too hard to improve the margin by giving model
too much degree of freedom such that the training margins are uniformly improved during training process, the predictability may be lost. A trade-off is thus necessary to balance the complexity of model and dataset
, otherwise one is doomed to meet Breiman's dilemma when the models arbitrarily increase the expression power.The example above shows that the expression power of models relative to the complexity of dataset, can be observed from the dynamics of normalized margins in training, instead of counting the number of parameters in neural networks. In the sequel, our main contributions are to make these precise by revisiting
the Rademacher complexity bounds with Lipschitz constants BID0 .• With the Lipschitz-normalized margins, a linear inequality is established between
training margin and test margin in Theorem 1. When both training and test normalized margin distributions undergo similar phase
transitions on increase-decrease during the training process, one may predict the generalization error based on the training margins as illustrated in FIG0 .• In a dual direction, one can define a quantile margin via the inverse of margin distribution
functions, to establish another linear inequality between the inverse quantile margins and the test margins as shown in Theorem 2. Quantile margin is far easier to tune in practice and enjoys a stronger prediction power exploiting
an adaptive selection of margins along model training.• In all cases, Breiman's dilemma may fail both of the methods above when dynamics of normalized training
margins undergo different phase transitions to that of test margins during training, where a uniform improvement of margins results in overfitting.Section 2 describes our method to derive the two linear inequalities of generalization bounds above.Extensive experimental results are shown in Section 3 and Appendix with basic CNNs, AlexNet, VGG, ResNet, and various datasets including CIFAR10, CIFAR100, and mini-Imagenet.
In this paper, we show that Breiman's dilemma is ubiquitous in deep learning, in addition to previous studies on Boosting algorithms.
We exhibit that Breiman's dilemma is closely related to the tradeoff between model expression power and data complexity.
A novel perspective on phase transitions in dynamics of Lipschitz-normalized margin distributions is proposed to inspect when the model has over-representation power compared to the dataset, instead of merely counting the number of parameters.
A data-driven early stopping rule by monitoring the margin dynamics is a future direction to explore.
Lipschitz semi-norm plays an important role in normalizing or regularizing neural networks, e.g. in GANs BID7 BID14 , therefore a more careful treatment deserves further pursuits.

assistant: text: Bregman's dilemma is shown in deep learning that improvement of margins of over-parameterized models may result in overfitting, and dynamics of normalized margin distributions are proposed to predict generalization error and identify such a dilemma.