SEVAL

Introduction

We find that current SSL algorithms struggle to perform well when exposed to imbalanced training data because the generated pseudo-labels tend to be biased toward the majority class. We closely examine pseudo-labeling for imbalanced SSL through the lens of statistical analysis and find that existing SSL algorithms, which generate pseudo-labels using heuristic strategies or uncalibrated model confidence, are unreliable.

To address this, we propose learning refinement and thresholding parameters from a holdout dataset in a class-balanced manner. Our pseudo-labeling strategies are theoretically superior to their counterparts, and our experiments show that SEVAL surpasses state-of-the-art SSL methods, delivering more accurate and effective pseudo-labels in various imbalanced SSL situations.

Installation

For Conda users, you can create a new Conda environment using

conda create -n seval python=3.7

after activating the environment with

source activate seval

try to install all the dependencies with

pip install -r requirements.txt

also install the conda environment for the jupyter notebook kernel.

python -m ipykernel install --user --name=seval

Training on CIFAR10-LT, CIFAR100-LT, and STL10-LT

Please use corresponding configuration files and select the appropriate hyper-parameters for varying experiment settings, as summarized in our paper. The datasets will be downloaded once the related code is excuted.

For example, running a standard FixMatch on CIFAR-10-LT:

python main.py --config-file configs/cifar10/fixmatch.yaml \
                DATASET.CIFAR10.NUM_LABELED_HEAD 500 \
                DATASET.CIFAR10.NUM_UNLABELED_HEAD 4000 \
                SEED 79 GPU_ID 0

Runing SEVAL with the same setting:

Firstly, estimate $\pi$ and $\tau$:

python main.py --config-file configs/cifar10/fixmatch_seval_estim.yaml \
                DATASET.CIFAR10.NUM_LABELED_HEAD 500 \
                DATASET.CIFAR10.NUM_UNLABELED_HEAD 4000 \
                SEED 79 GPU_ID 0

Then, standard training process following (which load the parameter learned):

python main.py --config-file configs/cifar10/fixmatch_seval_load.yaml \
                DATASET.CIFAR10.NUM_LABELED_HEAD 500 \
                DATASET.CIFAR10.NUM_UNLABELED_HEAD 4000 \
                SEED 79 GPU_ID 0

Or directly utilize additional validation data:

python main.py --config-file configs/cifar10/fixmatch_seval.yaml \
                DATASET.CIFAR10.NUM_LABELED_HEAD 500 \
                DATASET.CIFAR10.NUM_UNLABELED_HEAD 4000 \
                SEED 79 GPU_ID 0

Preparation for Semi-Aves

Download data and label, and place them in the following format.

├── data
│   ├── semi_fgvc
│   │   ├── trainval_images
│   │   ├── test
│   │   ├── u_train_in
│   │   ├── u_train_out
│   │   ├── l_train_val.txt
│   │   ├── u_train_in.txt
│   │   ├── u_train.txt
│   │   └── test.txt

Preparation for ImageNet-127

Download data (32x32), and place them in the following format.

├── data
│   ├── imagenet
│   │   ├── train_data_batch_1
│   │   ├── train_data_batch_2
│   │   ├── ...
│   │   └── val_data

cd data/imagenet/
python construct_small_imagenet_127.py

Acknowledgement

This code borrows from DASO.

Citation

If you find our work beneficial for your projects, please kindly consider citing it:

@article{li2026semi,
  title={Learning Label Refinement and Threshold Adjustment for Imbalanced Semi-Supervised Learning},
  author={Li, Zeju and Zheng, Ying-Qiu and Chen, Chen and Jbabdi, Saad},
  journal={Transactions on Machine Learning Research},
  year={2026}
}