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# System prepended metadata

title: Mitigating Source Biases - NeurIPS 2023 Rebuttal

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### Common Response
We thank all of the reviewers for their kind comments and feedback. Reviewers recognized the strengths of our paper, which we briefly reiterate before we dive into in-depth responses.
* **This is the first study of fairness in weak supervision** (Reviewers wmSM, UYjh, GLUc, 6e6W).
* Our approach offers **theoretical results** showing that (1) even when a dataset with ground-truth labels is fair, **a weakly-supervised counterpart can be arbitrarily biased** and (2) a finite-sample **recovery result for the correction algorithm** (Reviewers wmSM, UYjh, GLUc, 6e6W).
* **Strong empirical results on synthetic and real datasets** (Reviewers UYjh, GLUc).
* The proposed method is **compatible with the existing fair machine learning algorithms** (Reviewers wmSM, 6e6W).

We address two common questions before proceeding to individual responses.
* **On advantages and limitations of our formulation** (Reviewers UYjh,  6e6W): First, we highlight how **bias in weak supervision differs** from typical supervised learning settings. In supervised learning, datasets have inherent fairness violations and fairness-aware methods try to train a fair model from unfair data. Such methods typically use constrained optimization, which tries to fit the training data under a fairness constraint. This usually entails tradeoffs between fairness and performance. The type of unfairness we tackle is **different**. It is an artifact of the WS process (i.e., how the noisy labels are generated) and may not be inherent in datasets (Theorem 1 in our work characterizes a range of such scenarios). As a result, it is possible to **i) maintain the advantages of using WS (cheap labeling), ii) improve fairness, and iii) improve performance**. Our method uses a simple counterfactual mapping to seek to achieve all of these benefits simultaneously. 

    One potential drawback appears to be that high-quality estimation of the counterfactual mapping may be difficult. Fortunately, scenarios where this is the case are not common in weak supervision, since the standard setting involves using large unlabeled input datasets. This allows for small estimation error in the counterfactual mapping. As a result, we do not observe this limitation in any of the practical settings of interest. Even when this limitation is operative (i.e., when using small unlabeled datasets), it is still possible to overcome it by applying recent techniques in optimal transport [1,2] that enable improving the estimation of the counterfactual mapping. 
    
    This is done, for example, by exploiting a handful of matched _keypoints_. Such methods can be easily applied if practitioners have some intuition about group matchings (e.g. toxic comment matching in different languages)---which is often the case in weak supervision scenarios. 

* **On the relationship between the fairness and the number of labeling functions** (Reviewers UYjh, wmSM): Several reviewers hypothesize that diversifying label sources naturally resolves unfairness. Indeed, we had a similar belief early on in our study. Unfortunately, we ultimately observed the opposite scenario:  diversification can increase unfairness in practice. To demonstrate this idea, we conducted experiments varying the number of labeling functions and their parameters. We include one such case for synthetic data, depicting it in the figure in A.1. It shows that **in standard WS, increasing the number of LFs increases bias** as the accuracy improves. Meanwhile, **our approach improves fairness _and_ performance** when increasing the number of labeling functions.

[1] Gu, Xiang, et al. "Keypoint-guided optimal transport with applications in heterogeneous domain adaptation.", NeurIPS 2022.
    
[2] Panda, Nishant, et al. "Semi-supervised Learning of Pushforwards For Domain Translation & Adaptation." , arxiv 2023.
    
### Reviewer 1
We sincerely thank the reviewer for their kind words, constructive feedback, and useful suggestions.
* **On the number of labeling functions**: We discussed the relationship between the number of LFs, fairness, and performance in our common response. The result  shows that our method can resolve unfairness induced by weak supervision and improve performance, while existing weak supervision baselines lose fairness as they gain performance by increasing the number of LFs.
* **On improving individual LF fairness**: Indeed, if we could write fair labeling functions directly, the resulting datasets would be fair as well. Unfortunately, this is extremely difficult. Most labeling functions are small programs written by domain experts to express heuristic ideas. Ensuring that these programs are themselves fair requires substantial research progress in other fields. Similarly, other sources for labeling functions are pre-existing external knowledge bases, crowdworkers, pretrained models, and more. Here as well, ensuring fairness directly would be very hard. As a concrete example, labeling functions in our vision dataset experiments are generated by models pretrained on other datasets. In such situations, where we do not have access to the original dataset, it may not even be possible to directly modify LFs to be fair. Instead, **our method provides a generic and cheap approach to deal with the bias of label sources in weak supervision.** 

    Another idea is to simply demand that each labeling function used is created to be fair in the first place, throwing away any type of source of LFs that may not satisfy this property (including heuristics, pretrained models, etc.) Doing so would ensure fairness as well---but removes all of the benefits of weak supervision. In fact, crafting only fair labeling functions that lead to a satisfactory dataset may be more difficult than hand-labeling in the first place, which is the pain point weak supervision was designed to solve. The **value of our approach is that it is best-of-all-worlds**: we can continue to omnivorously use weak sources of signal, while handling bias with the additional technique we proposed. 
* **On the F1 score in Bank Marketing and CivilComments datasets**: As mentioned, Bank Marketing and CivilComments datasets have severe class imbalance (the proportion of positive class: 11.8% and 11.3%, respectively), which may lead more imbalance in the transport step, producing high odds to flip noisy labels towards dominant classes. A simple remedy is performing balanced sampling. We can selectively use the target group data points so that their noisy labels have a balanced class ratio. The table below shows the result with balanced transport in Bank Marketing data with LIFT. We observe that this simple idea can improve the F1 score while maintaining accuracy and fairness scores. In general, _any_ counterfactual mapping estimation method that considers class imbalance can be plugged in.
    
| Cond                                  | Acc           | F1            | DP Gap        | EO Gap        |
| ------------------------------------- | ------------- | ------------- | ------------- | ------------- |
| FS                                    | $0.912 \pm 0.000$ | $0.518 \pm 0.000$ | $0.128 \pm 0.000$ | $0.117 \pm 0.000$ |
| WS (Baseline)                         | $0.674 \pm  0.000$ | $0.258 \pm 0.000$ | $0.543 \pm 0.000$ | $0.450 \pm 0.000$ |
| SBM (w/o OT) + LIFT                   | $0.698\pm 0.000$ | $0.255 \pm 0.000$ | $0.088 \pm 0.000$ | $0.137 \pm 0.000$ |
| SBM (OT-L) + LIFT                     | $0.892 \pm 0.015$ | $0.305 \pm 0.015$ | $0.104 \pm 0.001$ | $0.121 \pm 0.019$ |
| SBM (OT-S) + LIFT                     | $0.698 \pm 0.011$ | $0.080 \pm 0.006$ | $0.109 \pm 0.017$ | $0.072 \pm 0.014$ |
| SBM (OT-S) + LIFT + Balanced sampling | $0.827 \pm 0.002$ | $0.498 \pm 0.003$ | $0.133 \pm 0.002$ | $0.077 \pm 0.002$ |

* **On error ranges**: Our proposed method is fairly simple and the effect of randomness is very limited. In repeated experiments with different seeds,  **we do not observe any significant deviations, but will provide error ranges from repeated runs in the updated draft.** We include several examples of these updated results in the following:
    
    * Synthetic data experiments (Section 5.2. Figure 4): In rebuttal supplementary A.2, we show 95% confidence intervals, which are from 10 repetitions with different seeds. Unsurprisingly, the deviation rapidly drops as n increases.
    * Real data experiments (Section 5.1.): We reported the mean of 5 repeated experiment results in Section 5.1. We will include error ranges in our updated manuscript. Examples include:
#### Adult
    
| Cond                | Acc           | F1            | DP Gap        | EO Gap        |
| ------------------- | ------------- | ------------- | ------------- | ------------- |
| FS                  | $0.824 \pm 0.000$ | $0.564 \pm 0.000$ | $0.216 \pm 0.000$ | $0.331 \pm 0.000$ |
| WS (Baseline)       | $0.717 \pm 0.000$ | $0.587 \pm 0.000$ | $0.475 \pm 0.000$ | $0.325 \pm 0.000$ |
| SBM (w/o OT)        | $0.720 \pm 0.000$ | $0.592 \pm 0.000$ | $0.439 \pm 0.000$ | $0.273 \pm 0.000$ |
| SBM (OT-L)          | $0.560 \pm 0.000$ | $0.472 \pm 0.000$ | $0.893 \pm 0.000$ | $0.980 \pm 0.000$ |
| SBM (OT-S)          | $0.723 \pm 0.003$ | $0.590 \pm 0.003$ | $0.429 \pm 0.010$ | $0.261 \pm 0.005$ |
| SBM (w/o OT) + LIFT | $0.704 \pm 0.000$ | $0.366 \pm 0.000$ | $0.032 \pm 0.000$ | $0.192 \pm 0.000$ |
| SBM (OT-L) + LIFT   | $0.700 \pm 0.017$ | $0.520 \pm 0.005$ | $0.015 \pm 0.015$ | $0.138 \pm 0.020$ |
| SBM (OT-S) + LIFT   | $0.782 \pm 0.043$ | $0.448 \pm 0.015$ | $0.000 \pm 0.000$ | $0.178 \pm 0.036$ |

#### Bank
| Cond                                  | Acc           | F1            | DP Gap        | EO Gap        |
| ------------------------------------- | ------------- | ------------- | ------------- | ------------- |
| FS                                    | $0.912 \pm 0.000$ | $0.518 \pm 0.000$ | $0.128 \pm 0.000$ | $0.117 \pm 0.000$ |
| WS (Baseline)                         | $0.674 \pm 0.000$ | $0.258 \pm 0.000$ | $0.543 \pm 0.000$ | $0.450 \pm 0.000$|
| SBM (w/o OT)                          | $0.876 \pm 0.000$ | $0.550 \pm 0.000$ | $0.106 \pm 0.000$ | $0.064 \pm 0.000$ |
| SBM (OT-L)                            | $0.892 \pm 0.003$ | $0.304 \pm 0.145$ | $0.095 \pm 0.054$ | $0.124 \pm 0.032$ |
| SBM (OT-S)                            | $0.847 \pm 0.014$ | $0.515 \pm 0.016$ | $0.122 \pm 0.006$ | $0.080 \pm 0.013$ |
| SBM (w/o OT) + LIFT                   | $0.698 \pm 0.000$ | $0.255 \pm 0.000$ | $0.088 \pm 0.000$ | $0.137 \pm 0.000$ |
| SBM (OT-L) + LIFT                     | $0.892 \pm 0.015$ | $0.305 \pm 0.015$ | 0.104 (0.001) | 0.121 (0.019) |
| SBM (OT-S) + LIFT                     | 0.698 (0.011) | 0.080 (0.006) | 0.109 (0.017) | 0.072 (0.014) |
| SBM (OT-S) + LIFT + Balanced sampling | 0.827 (0.002) | 0.498 (0.003) | 0.133 (0.002) | 0.077 (0.002) |


#### HateXplain
| Cond          | Acc           | F1            | DP Gap        | EO Gap        |
| ------------- | ------------- | ------------- | ------------- | ------------- |
| FS            | $0.698 \pm 0.000$ | $0.755 \pm 0.000$ | $0.238 \pm 0.000$ | $0.121 \pm 0.000$ |
| WS (Baseline) | $0.584 \pm 0.000$ | $0.590 \pm 0.000$  | $0.170 \pm 0.000$  | $0.133 \pm 0.000$ |
| SBM (w/o OT)  | $0.592 \pm 0.000$ | $0.637 \pm 0.000$ | $0.159 \pm 0.000$ | $0.138 \pm 0.000$ |
| SBM (OT-L)    | $0.670 \pm 0.000$ | $0.606 \pm 0.000$ | $0.120 \pm 0.000$ | $0.101 \pm 0.000$ |
| SBM (OT-S)    | $0.612 \pm 0.001$ | $0.687 \pm 0.001$ | $0.072 \pm 0.004$ | $0.037 \pm 0.003$ |
    
#### UTKFace
| Cond          | Acc           | F1            | DP Gap        | EO Gap        |
| ------------- | ------------- | ------------- | ------------- | ------------- |
| FS            | $0.810 \pm 0.000$  | $0.801 \pm 0.000$ | $0.133 \pm 0.000$ | $0.056 \pm 0.000$ |
| WS (Baseline) | $0.791 \pm 0.000$ | $0.791 \pm 0.000$ | $0.172 \pm 0.000$ | $0.073 \pm 0.000$ |
| SBM (w/o OT)  | $0.797 \pm 0.000$ | $0.790 \pm 0.000$  | $0.164 \pm 0.000$ | $0.077 \pm 0.000$ |
| SBM (OT-L)    | $0.800 \pm 0.001$ | $0.793 \pm 0.002$ | $0.135 \pm 0.005$ | $0.043 \pm 0.005$ |
| SBM (OT-S)    | $0.804 \pm 0.002$ | $0.798 \pm 0.002$ | $0.130 \pm 0.008$ | $0.041 \pm 0.007$ |


### Reviewer 2
We are grateful to the reviewer for noting the strengths of our work and providing useful questions and comments.
* **On the potential drawbacks of the proposed method in the programmatic WS settings**: A possible drawback of the proposed method is that it requires high quality estimation of Monge mapping (or any counterfactual mapping.) Fortunately, as we described in the general response, in the typical weak supervision settings, we have access to massive unlabeled datasets which enable reliable estimation in these procedures. Better yet, we have found that our method works well even with relatively small datasets ($n \leq 10^4$, e.g. the Wrench Mushroom dataset ($n=6499$)). 

    If the available unlabeled data is genuinely insufficient, our technique may struggle. To address this, we note that there are ways to overcome this limitation as well. For example, there are techniques that can dramatically reduce the amount of data that is needed in optimal transport. Such techniques involve the use of _keypoints_ (pairs of matched points) [1]. Domain experts could craft such pairs, enabling optimal transport, and ultimately our entire pipeline, to operate in the very low data regime. 

* **On the possibility that diverse label sources resolve unfairness naturally**: As mentioned in our common resonse, this appears to be a plausible approach towards mitigating unfairness. Unfortunately, diverse sources may not in fact behave in this way---further motivating the use of our approach. For example, our synthetic experiment in A.1. shows that diverse label sources can make fairness _worse_. In this setting, label sources that have high accuracy centers around the major clusters have higher weights in voting as an outcome of label model learning in weak supervision. Thus, increasing the number of LFs (diversifying) can be beneficial in finding such centers, and yields improved performance. However, it makes disadvantaged groups perform worse, since they are dislocated from high accuracy centers, leading to unfairness. SBM not only resolves such issues, but also further improves the performance gain by bringing back the dislocated data points around the high accuracy centers.

[1] Gu, Xiang, et al. "Keypoint-guided optimal transport with applications in heterogeneous domain adaptation.", NeurIPS 2022.
    

    
### Reviewer 3
We thank the reviewer for the suggestions and noting the strengths of our work. We have added the suggested reference.
* **On modeling assumptions**. Indeed, our label model _may appear_, at first glance, to require strong assumptions. However, in fact it **has weaker assumptions compared to previous weak supervision models.** We describe the differences versus two popular weak supervision models next. Common models use one of the following assumptions:
**i) Ising model assumption [1, 2, 3]**. Here, the model is given by 
$$P(\lambda^1, \ldots, \lambda^m,y) = \frac{1}{Z}\exp(\theta_y y + \sum_{j=1}^m \theta_j \lambda^j y)$$ 
Here, the $\lambda$'s are labeling functions, $y$ is the latent true label, $Z$ is a partition function (a normalizing constant), the $\theta$'s are the model parameters.

    This label model is based on the Ising pairwise interactions model. In this model, parameters are estimated globally, which means that **the label model requires uniform accuracy in the entire feature space---a strong assumption.** 
    
    In other words, regardless of the point $x \in \mathcal{X}$ whose label is being estimated, the assumption states that the labeling function accuracies are identical. Naturally, this is not a realistic assumption. Indeed, in practice, it has been observed that LF accuracy strongly depends on the input features, as certain parts of the feature space are invariably more challenging to label than others.
    
    **ii) Strong smoothness leading to a partitionable feature space [4]**
$$P_x(\lambda^1, \ldots, \lambda^m, y) = \frac{1}{Z}\exp(\theta_y y + \sum_{j=1}^m \theta_{j,x} \lambda^j(x) y)$$
This model assumes that accuracy depends on the input feature $x$. In fact, in its most basic form, each $x$ has a separate model (with parameters given by $\theta_x$). However, since we can observe only one sample for each $x$, recovering parameters $\theta_{j, x}$ is impossible. Thus the authors in [4] instead assume strong smoothness: that is, **that the feature space can be partitioned and the accuracy in each partition is uniform.** This is still a strong assumption. While this model incorporated the input feature space, nevertheless accuracy tends to drop as data points get far from high accuracy centers---even within parts. This observation led to our newly-proposed model. 

    **iii) Proposed model**
$$P(\lambda^1(z), \ldots, \lambda^m(z), y) = \frac{1}{Z}\exp\left(\theta_yy + \sum_{j=1}^m \frac{\theta_{j}}{1+d(x^{\text{center}_j}, g_k(z))} \lambda^j(g_k(z)) y\right)$$
Our label model captures the phenomenon in Figure 2 of our draft, which shows that the accuracy of each LF drops as it moves away from its center. This means that our model neither require universal uniformity, as (i) does, nor per-part uniformity, as (ii) does. It makes the much weaker assumption of the existence of a center point where LFs are most accurate---which indeed matches practical LF scenarios, especially for LFs that are programs expressing heuristics. As a result, **our model not only has less restrictive assumptions, but also provides a framework that interprets unfairness as drift away from centers as an outcome of a group transformation.** 
    
    
[1] Ratner, Alexander, et al. "Snorkel: Rapid training data creation with weak supervision." VLDB 2017.
[2] Fu, Daniel, et al. "Fast and three-rious: Speeding up weak supervision with triplet methods." ICML 2020.
[3] Ratner, Alexander, et al. "Training complex models with multi-task weak supervision." AAAI 2019.
[4] Chen, Mayee F., et al. "Shoring up the foundations: Fusing model embeddings and weak supervision.", UAI 2022.


### Reviewer 4
We appreciate the reviewer for the thoughtful comments and reference. We will add the suggested paper in our updated draft.
* **On the choice of label models**: A remarkable property of our label model is that **it is only used for one step in our overall pipeline: mitigating source bias**. Once this step is complete, any other label model can be used for aggregating sources into pseudolabels. Our WRENCH experiment (Table 5) is conducted with the newly-introduced Hyper Label Model [1] after applying SBM.
* **On the trade-off between performance and fairness in ML models**: We  discuss this question in our common response and provide more detail here.  The trade-off between performance and fairness in machine learning models is usually caused by inherent fairness violations in datasets. Models trained on such datasets are unfair, and so fairness techniques seek to mitigate this, but ultimately face constraints. 

    On the other hand, **our method tackles bias induced by creating labeled datasets via weak supervision**. Weak supervision allows for the efficient creation of labeled datasets by using multiple noisy sources of signal for labels. In addition to being noisy, these sources are often far more biased than hand-labeled datasets. As a result, a dataset created through weak supervision may prove to be highly unfair even if its counterpart with ground-truth labels was perfectly fair. 

    This problem is also an opportunity. Our approach is to model where bias is induced within weak supervision sources, remove this bias, and as a result, improve both fairness and performance, while keeping the advantage of using weak supervision---exploiting cheap label sources omnivorosly.

[1] Wu, Renzhi, et al. "Learning Hyper Label Model for Programmatic Weak Supervision.", ICLR 2022.