INTERPRETABLE COUNTING FOR VISUAL QUESTION ANSWERING
Questions that require counting a variety of objects in images remain a major challenge in visual question answering (VQA). The most common approaches to VQA involve either classifying answers based on fixed length representations of both the image and question or summing fractional counts estimated from each section of the image. In contrast, we treat counting as a sequential decision process and force our model to make discrete choices of what to count. Specifically, the model sequentially selects from detected objects and learns interactions between objects that influence subsequent selections. A distinction of our approach is its intuitive and interpretable output, as discrete counts are automatically grounded in the image. Furthermore, our method outperforms the state of the art architecture for VQA on multiple metrics that evaluate counting.
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Summary:
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This work introduces a discrete and interpretable model for answering visually grounded counting questions. The proposed model executes a sequential decision process in which it 1) selects an image region to "add to the count" and then 2) updates the likelihood of selecting other regions based on their relationships (defined broadly) to the selected region. After substantial module pre-trianing, the model is trained end-to-end with the REINFORCE policy gradient method (with the recently proposed self-critical sequence training baseline). Compared to existing approaches for counting (or VQA in general), this approach not only produces lower error but also provides a more human-intuitive discrete, instance-pointing representation of counting. 
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Preliminary Evaluation:
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The paper presents an interesting approach that seems to outperform existing methods. More importantly in my view, the model treats counting as a discrete human-intuitive process. The presentation and experiments are okay overall but I have a few questions and requests below that I feel would strengthen the submission.
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Strengths:
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- I generally agree with the authors that approaching counting as a region-set selection problem provides an interpretable and human-intuitive methodology that seems more appropriate than attentional or monolithic approaches. 
- To the best of my knowledge, the writing does a good job of placing the work in the context of existing literature.
- The dataset construction is given appropriate attention to restrict its instances to counting questions and will be made available to the public.
- The model outperforms existing approaches given the same visual and linguistic inputs / encodings. While I find improvements in RMSE a bit underwhelming, I'm still generally positive about the results given the improved accuracy and human-intuitiveness of the grounded outputs.
- I appreciated the analysis of the effect of "commonness" and think it provides interesting insight into the generalization of the proposed model.
- Qualitative examples are interesting.
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Weaknesses:
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- There is a lot going on in this paper as far as model construction and training procedures go. In its current state, many of the details are pushed to the supplement such that the main paper would be insufficient for replication. The authors also do not promise code release. 
- Maybe it is just my unfamiliarity with it, but the caption grounding auxiliary-task feels insufficiently introduced in the main paper.  I also find it a bit discouraging that the details of joint training is regulated to the supplementary material, especially given that the UpDown is not using it! I would like to see an ablation of the proposed model without joint training.
- Both the IRLC and SoftCount models are trained with objectives that are aware of the ordinal nature of the output space (such that predicting 2 when the answer is 20 is worse than predicting 19). Unfortunately, the UpDown model is trained with cross-entropy and lacks access to this notion. I believe that this difference results in the large gap in RMSE between IRLC/SoftCount and UpDown. Ideally an updated version of UpDown trained under an order-aware loss would be presented during the rebuttal period. Barring that due to time constraints, I would otherwise like to see some analysis to explore this difference, maybe checking to see if UpDown is putting mass in smooth blobs around the predicted answer (though there may be better ways to see if UpDown has captured similar notions of output order as the other models).
- I would like to see a couple of simple baselines evaluated on HowMany-QA. Specifically, I think the paper would be stronger if results were put in context with a question only model and a model which just outputs the mean training count. Inter-human agreement would also be interesting to discuss (especially for high counts).
- The IRLC model has a significantly large (4x) capacity scoring function than the baseline methods. If this is restricted, do we see significant changes to the results?
- This is a relatively mild complaint. This model is more human-intuitive than existing approaches, but when it does make an error by selecting incorrect objects or terminating early, it is no more transparent about the cause of these errors than any other approach. As such, claims about interpretability should be made cautiously.  
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Curiosities:
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- In my experience, Visual Genome annotations are often noisy, with many different labels being applied to the same object in different images. For per-image counts, I don't imagine this will be too troubling but was curious if you ran into any challenges.
- It looks like both IRLC and UpDown consistently either get the correct count (for small counts) or underestimate. This is not the Gaussian sort of regression error that we might expect from a counting problem. 
- Could you speak to the sensitivity of the proposed model with respect to different loss weightings? I saw the values used in Section B of the supplement and they seem somewhat specific. 
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Minor errors:
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[5.1 end of paragraph 2] 'that accuracy and RSME and not' -> 'that accuracy and RSME are not'
[Fig 9 caption] 'The initial scores are lack' -> 'The initial scores lack'