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Title: Efficient Augmentation via Data Subsampling

Paper content:
Data augmentation is commonly used to encode invariances in learning methods.
However, this process is often performed in an inefficient manner, as artificial examples are created by applying a number of transformations to all points in the training set.
The resulting explosion of the dataset size can be an issue in terms of storage and training costs, as well as in selecting and tuning the optimal set of transformations to apply.
In this work, we demonstrate that it is possible to significantly reduce the number of data points included in data augmentation while realizing the same accuracy and invariance benefits of augmenting the entire dataset.
We propose a novel set of subsampling policies, based on model influence and loss, that can achieve a 90% reduction in augmentation set size while maintaining the accuracy gains of standard data augmentation.
Data augmentation is a process in which the training set is expanded by applying class-preserving transformations, such as rotations or crops for images, to the original data points.
This process has become an instrumental tool in achieving state-of-the-art accuracy in modern machine learning pipelines.
Indeed, for problems in image recognition, data augmentation is a key component in achieving nearly all state-of-the-art results BID6 BID10 Graham, 2014; Sajjadi et al., 2016) .
Data augmentation is also a popular technique because of its simplicity, particularly in deep learning applications, where applying a set of known invariances to the data is often more straightforward than trying to encode this knowledge directly in the model architecture.However, data augmentation can be an expensive process, as applying a number of transformations to the entire dataset may increase the overall size of the dataset by orders of magnitude.
For example, if applying just 3 sets of augmentations (e.g., translate, rotate, crop), each with 4 possible configurations, the dataset can easily grow by a factor of 12 (if applied independently), all the way to 64x (if applied in sequence).
While this may have some benefits in terms of overfitting, augmenting the entire training set can also significantly increase data storage costs and training time, which can scale linearly or superlinearly with respect to the training set size.
Further, selecting the optimal set of transformations to apply to a given data point is often a non-trivial task.
Applying transformations not only takes processing time, but also frequently requires some amount of domain expertise.
Augmentations are often applied heuristically in practice, and small perturbations are expected (but not proven) to preserve classes.
If more complex augmentations are applied to a dataset, they may have to be verified on a per-sample basis.In this work, we aim to make data augmentation more efficient and user-friendly by identifying subsamples of the full dataset that are good candidates for augmentation.
In developing policies for subsampling the data, we draw inspiration from the virtual support vector (VSV) method, which has been used for this purpose in the context of SVMs BID4 BID9 .
The VSV method attempts to create a more robust decision surface by augmenting only the samples that are close to the margin-i.e., the support vectors.
The motivation is intuitive: if a point does not affect the margin, then any small perturbation of that point in data space will likely yield a point that is again too far from the margin to affect it.
The method proceeds by applying classpreserving data augmentations (e.g., small perturbations) to all support vectors in the training set.
The SVM is then retrained on the support vector dataset concatenated with the augmented dataset, and the end result is a decision surface that has been encoded with transformation invariance while augmenting many fewer samples than found in the full training set.Although proven to be an effective approach for SVMs, methods utilizing support vectors may not generalize well to other classifiers.
Therefore, in this work, we aim to develop policies that can effectively reduce the augmentation set size while applying to a much broader class of models.
A key step in developing these policies is to determine some metric by which to rank the importance of data points for augmentation.
We build policies based on two key metrics.
First, we make a natural generalization of the VSV method by measuring the loss induced by a training point.
Second, we explore using the influence of a point as an indicator of augmentation potential.
Influence functions, originating from robust statistics, utilize more information than loss (i.e., residuals) alone, as they take into account both leverage and residual information.The contributions of this paper are as follows.
First, we demonstrate that it is typically unnecessary to augment the entire dataset to achieve high accuracy-for example, we can maintain 99.86% or more of the full augmentation accuracy while only augmenting 10% of the dataset in the case of translation augmentations, and we observe similar behavior for other augmentations.
Second, we propose several policies to select the subset of points to augment.
Our results indicate that policies based off of training loss or model influence are an effective strategy over simple baselines, such as random sampling.
Finally, we propose several modifications to these approaches, such as sample reweighting and online learning, that can further improve performance.
Our proposed policies are simple and straightforward to implement, requiring only a few lines of code.
We perform experiments throughout on common benchmark datasets, such as MNIST (LeCun et al., 1998) , CIFAR10 (Krizhevsky, 2009) , and NORB (LeCun et al., 2004) .
In this paper, we demonstrate that not all training points are equally useful for augmentation, and we propose simple policies that can select the most viable subset of points.
Our policies, based on notions of training loss and model influence, are widely applicable to general machine learning models.
Obtaining access to an augmentation score vector can be obtained in only one training cycle on the original data (e.g., a fixed cost), yet the potential improvements in augmented training can scale superlinearly with respect to the original dataset size.
With many fewer data points to augment, the augmentations themselves can be applied in a more efficient manner in terms of compute and expert oversight.
At an extreme, they can be specialized on a per-example basis.A natural area of future work is to explore subset selection policies that take the entire subset into account, rather than the greedy policies described.
For example, even if two samples may independently have large leave-one-out influence, it may be the case that these points influence each other and leave-one-out influence may be an overestimate (e.g., consider the case of two identical samples).
Including second-order information or encouraging subset diversity 4 may therefore help to improve performance even further.

assistant: text: The authors propose to use influence or loss-based methods to select a subset of points to use in augmenting data sets for training models where the loss is additive over data points.