Paper:
Due to the success of residual networks (resnets) and related architectures, shortcut connections have quickly become standard tools for building convolutional neural networks. The explanations in the literature for the apparent effectiveness of shortcuts are varied and often contradictory. We hypothesize that shortcuts work primarily because they act as linear counterparts to nonlinear layers. We test this hypothesis by using several variations on the standard residual block, with different types of linear connections, to build small (100k-1.2M parameter) image classification networks. Our experiments show that other kinds of linear connections can be even more effective than the identity shortcuts. Our results also suggest that the best type of linear connection for a given application may depend on both network width and depth.

Review:
This paper performs an analysis of shortcut connections in ResNet-like architectures. The authors hypothesize that the success of shortcut connections comes from the combination of linear and non-linear features at each layer and propose to substitute the identity shortcuts with a convolutional one (without non-linearity). This alternative is referred to as tandem block. Experiments are performed on a variety of image classification tasks such as CIFAR-10, CIFAR-100, SVHN and Fashion MNIST.
The paper is well structured and easy to follow. The main contribution of the paper is the comparison between identity skip connections and skip connections with one convolutional layer.
My main concerns are related to the contribution of the paper and experimental pipeline followed to perform the comparison. First, the idea of having convolutional shortcuts was already explored in the ResNet paper (see https://arxiv.org/pdf/1603.05027.pdf). Second, given Figures 3-4-5-6, it would seem that the authors are monitoring the performance on the test set during training. Moreover, results on Table 2 are reported as the ones with “the highest test accuracy achieved with each tandem block”. Could the authors give more details on how the hyperparameters of the architectures/optimization were chosen and provide more information on how the best results were achieved?
In section 3.5, the authors mention that batchnorm was not useful in their experiments, and was more sensitive to the learning rate value. Do the authors have any explanation/intuition for this behavior?
In section 4, authors claim that their results are competitive with the best published results for a similar number of parameters. It would be beneficial to add the mentioned best performing models in Table 2 to back this statement. Moreover, it seems that in some cases such as SVHN the differences between all the proposed blocks are too minor to draw any strong conclusions. Could those differences be due to, for example, luck in picking the initialization seed? How many times was each experiment run? If more than once, what was the std?
The experiments were performed on relatively shallow networks (8 to 26 layers). I wonder how the conclusions drawn scale to much deeper networks (of 100 layers for example) and on larger datasets such as ImageNet.
Figures 3-5 are not referenced nor discussed in the text.
Following the design of the tandem blocks proposed in the paper, I wonder why the tandem block B3x3(2,w) was not included.
Finally, it might be interesting to initialize the convolutions in the shortcut connections with the identity, and check what they have leant at the end of the training.
Some typos that the authors might want to fix:
- backpropegation -> backpropagation (Introduction, paragraph 3)
- dropout is a kind of regularization as well (Introduction, second to last paragraph)
- nad -> and (Sect 3.1. paragraph 1)