Domain Shift Research Articles

Aligning Correlation Information for Domain Adaptation in Action Recognition.

Domain adaptation (DA) approaches address domain shift and enable networks to be applied to different scenarios. Although various image DA approaches have been proposed in recent years, there is limited research toward video DA. This is partly due to the complexity in adapting the different modalities of features in videos, which includes the correlation features extracted as long-range dependencies of pixels across spatiotemporal dimensions. The correlation features are highly associated with action classes and proven their effectiveness in accurate video feature extraction through the supervised action recognition task. Yet correlation features of the same action would differ across domains due to domain shift. Therefore, we propose a novel adversarial correlation adaptation network (ACAN) to align action videos by aligning pixel correlations. ACAN aims to minimize the distribution of correlation information, termed as pixel correlation discrepancy (PCD). Additionally, video DA research is also limited by the lack of cross-domain video datasets with larger domain shifts. We, therefore, introduce a novel HMDB-ARID dataset with a larger domain shift caused by a larger statistical difference between domains. This dataset is built in an effort to leverage current datasets for dark video classification. Empirical results demonstrate the state-of-the-art performance of our proposed ACAN for both existing and the new video DA datasets.

Dynamic Weighting Translation Transfer Learning for Imbalanced Medical Image Classification.

Medical image diagnosis using deep learning has shown significant promise in clinical medicine. However, it often encounters two major difficulties in real-world applications: (1) domain shift, which invalidates the trained model on new datasets, and (2) class imbalance problems leading to model biases towards majority classes. To address these challenges, this paper proposes a transfer learning solution, named Dynamic Weighting Translation Transfer Learning (DTTL), for imbalanced medical image classification. The approach is grounded in information and entropy theory and comprises three modules: Cross-domain Discriminability Adaptation (CDA), Dynamic Domain Translation (DDT), and Balanced Target Learning (BTL). CDA connects discriminative feature learning between source and target domains using a synthetic discriminability loss and a domain-invariant feature learning loss. The DDT unit develops a dynamic translation process for imbalanced classes between two domains, utilizing a confidence-based selection approach to select the most useful synthesized images to create a pseudo-labeled balanced target domain. Finally, the BTL unit performs supervised learning on the reassembled target set to obtain the final diagnostic model. This paper delves into maximizing the entropy of class distributions, while simultaneously minimizing the cross-entropy between the source and target domains to reduce domain discrepancies. By incorporating entropy concepts into our framework, our method not only significantly enhances medical image classification in practical settings but also innovates the application of entropy and information theory within deep learning and medical image processing realms. Extensive experiments demonstrate that DTTL achieves the best performance compared to existing state-of-the-art methods for imbalanced medical image classification tasks.

Chemical fault diagnosis network based on single domain generalization

Recent developments in fault diagnosis have leveraged domain generalization to address the issue of domain shift. Most existing methods focus on learning domain-invariant representations from multiple source domains. However, collecting valuable fault samples from varying operational conditions is challenging, and it is common for available data to originate from a single operational condition. Thus, this paper introduces a Multi-scale generative and adversarial Metric networks (MGAMN) for Chemical Process Fault Diagnosis. To enhance model generalization, a domain generation module was developed to create augmented domains with significant distributional differences from the source domain. The diagnostic task module then extracts domain-invariant features from both the source and augmented domains. A multi-scale generation strategy is established, utilizing multi-scale deep separable convolutions (Dsc) to ensure that the generated samples contain rich state information. Additionally, an adversarial training and metric learning strategy is designed to learn generalized features capable of resisting unknown domain shifts. Extensive diagnostic experiments on the non-isothermal continuous stirred tank reactor (CSTR) and the Tennessee Eastman Process (TEP) chemical datasets validate the effectiveness of the proposed method. Moreover, ablation studies confirm the effectiveness of the proposed modular strategy, demonstrating significant potential for practical applications.

Many birds, one stone: Medical image segmentation with multiple partially labeled datasets

Medical image segmentation is fundamental in the field of medical image analysis and has wide clinical applications in disease diagnosis and surgical planning etc. Current prevalent solution is to train a deep network in a fully supervised way with a large-scale fully labeled dataset. However, due to the high labor cost and requirement on medical expertise, such dataset is always absent. Instead, there are multiple partially labeled datasets which are originally established for specific purposes. To make full use of these partially labeled datasets, we propose a novel partially supervised segmentation network, named PSSNet, which consists of a task-specific feature learning network followed by a cross-task attention module (xTA) to exploit task dependencies to enhance task-specific features. To solve the challenges raised by unlabeled classes and domain shift across datasets, we propose an adversarial self-training strategy. We conduct experiments on two medical image segmentation tasks. One is the fine-grained fundus image segmentation aiming to simultaneously segment four-class lesions, OD and OC, and vessels. Validation on seven datasets demonstrates that our PSSNet performs the best among three baselines and three state-of-the-arts. The other is the multiple abdominal organ segmentation in CT images. Our PSSNet is trained on three partially labeled datasets, i.e., LiTS, KiTS and Spleen. Validation on one fully labeled dataset, i.e., BTCV, demonstrates that our PSSNet achieves better performances than four state-of-the-arts. The code is publicly available at https://github.com/CVIU-CSU/PSSNet.

DISSECT: deep semi-supervised consistency regularization for accurate cell type fraction and gene expression estimation.

Cell deconvolution is the estimation of cell type fractions and cell type-specific gene expression from mixed data. An unmet challenge in cell deconvolution is the scarcity of realistic training data and the domain shift often observed in synthetic training data. Here, we show that two novel deep neural networks with simultaneous consistency regularization of the target and training domains significantly improve deconvolution performance. Our algorithm, DISSECT, outperforms competing algorithms in cell fraction and gene expression estimation by up to 14 percentage points. DISSECT can be easily adapted to other biomedical data types, as exemplified by our proteomic deconvolution experiments.

A robust quality estimation method for medical image segmentation with small datasets

The quality check of automatic image analysis results is a necessity to eliminate poor outcomes. There are few work on segmentation quality estimation when there is no reference mask available. Generative-based and regression approaches generally have strict assumptions on shape/contrast of anatomical structures, which may fail when there are abnormalities in images or in the presence of domain shift. Ensemble approach promises more generalisability to various types of images; however, they are costly to train. Also, none of these methods were designed for small datasets. To address these shortcomings, this paper presents a segmentation quality estimation method for small datasets with arbitrary dimensions, which is validated with 2D, 3D and 4D image datasets with roughly 20 training images, and on the segmentation of retinal vessel and left atrium segmentation. Although our method uses an ensemble for image segmentation, its design reduces its parameter size. We describe possible scenarios relating the amount of agreement across the base models’ outputs in an ensemble to quality scores; then, present a technique to deal with high quality score estimation for poor segmentation as a result of the base models to largely agree on mistakes. We assess the performance of our method in the presence of different sources of domain shift, and compare it with methods selected from the aforementioned approaches. We found robust quality score estimation, generalisable to different datasets. Our code would be available upon acceptance.

Privacy-preserving federated transfer learning for defect identification from highly imbalanced image data in additive manufacturing

Defect identification is a crucial task for process monitoring and quality evaluation in additive manufacturing (AM). Deep learning (DL) has shown great potential for diverse fields, but some challenges have hindered the application in AM process monitoring. Firstly, DL-based methods are driven by big data and require a large number of training data. However, in reality, defective data is often rare, so different AM manufacturers can collaborate to train a global model for detecting defects. Nevertheless, due to privacy concerns and conflicts of interest, data-sharing between different manufacturers might be dangerous. Additionally, the heterogeneities among manufacturers’ data leads to the domain shift, making it difficult to obtain a well-generalized model. Moreover, the imbalance issue of powder-spreading defects seriously damages the performance of the defect recognition. In this paper, we proposed FTLAM, a federated transfer learning method, to address the above issues. Concretely, to solve the data privacy concerns, federated learning (FL) is utilized to collaboratively train a global model between different AM manufacturers without exchanging original data. Furthermore, a client-oriented transfer method is proposed to mitigate the heterogeneities across multiple clients and improve the model's generalization. Meanwhile, a multi-loss joint optimization approach is designed to alleviate the data imbalance. Extensive experiments on laser powder bed fusion (LPBF) image datasets of powder-spreading have demonstrated that our FTLAM can obtain satisfactory performance of powder spreading defect identification.

Optimizing latent graph representations of surgical scenes for unseen domain generalization.

Advances in deep learning have resulted in effective models for surgical video analysis; however, these models often fail to generalize across medical centers due to domain shift caused by variations in surgical workflow, camera setups, and patient demographics. Recently, object-centric learning has emerged as a promising approach for improved surgical scene understanding, capturing and disentangling visual and semantic properties of surgical tools and anatomy to improve downstream task performance. In this work, we conduct a multicentric performance benchmark of object-centric approaches, focusing on critical view of safety assessment in laparoscopic cholecystectomy, then propose an improved approach for unseen domain generalization. We evaluate four object-centric approaches for domain generalization, establishing baseline performance. Next, leveraging the disentangled nature of object-centric representations, we dissect one of these methods through a series of ablations (e.g., ignoring either visual or semantic features for downstream classification). Finally, based on the results of these ablations, we develop an optimized method specifically tailored for domain generalization, LG-DG, that includes a novel disentanglement loss function. Our optimized approach, LG-DG, achieves an improvement of 9.28% over the best baseline approach. More broadly, we show that object-centric approaches are highly effective for domain generalization thanks to their modular approach to representation learning. We investigate the use of object-centric methods for unseen domain generalization, identify method-agnostic factors critical for performance, and present an optimized approach that substantially outperforms existing methods.

Target Oriented Dynamic Adaption for Cross-Domain Few-Shot Learning

Few-shot learning has achieved satisfactory progress over the years, but these methods implicitly hypothesize that the data in the base (source) classes and novel (target) classes are sampled from the same data distribution (domain), which is often invalid in reality. The purpose of cross-domain few-shot learning (CD-FSL) is to successfully identify novel target classes with a small quantity of labeled instances on the target domain under the circumstance of domain shift between the source domain and the target domain. However, in CD-FSL, the knowledge learned by the network on the source domain often suffers from the situation of inadaptation when it is transferred to the target domain, since the instances on the source and target domains do not obey the same data distribution. To surmount this problem, we propose a Target Oriented Dynamic Adaption (TODA) model, which uses a tiny amount of target data to orient the network to dynamically adjust and adapt during training. Specifically, this work proposes a domain-specific adapter to ameliorate the network inadaptability issues in transfer to the target domain. The domain-specific adapter can make the extracted features more specific to the tasks in the target domain and reduce the impact of tasks in the source domain by combining them with the mainstream backbone network. In addition, we propose an adaptive optimization method in the network optimization process, which assigns different weights according to the importance of different optimization tasks. Extensive experiments on several benchmark datasets demonstrate the effectiveness of our TODA method.

Recognition of a Single β-D-Xylopyranose Molecule by Xylanase GH11 from Thermoanaerobacterium saccharolyticum

The endo-β-1,4-xylanase glycosyl hydrolase (GH11) decomposes the backbone of xylan into xylooligosaccharides or xylose. These enzymes are important for industrial applications in the production of biofuel, feed, food, and value-added materials. β-D-xylopyranose (XYP, also known as β-D-xylose) is the fundamental unit of the substrate xylan, and understanding its recognition is fundamental for the initial steps of GH11’s molecular mechanism. However, little is known about the recognition of a single XYP molecule by GH11. In this study, the crystal structures of GH11 from Thermoanaerobacterium saccharolyticum (TsaGH11) complexed with an XYP molecule were determined at a resolution of 1.7–1.9 Å. The XYP molecule binds to subsite −2 of the substrate-binding cleft. The XYP molecule is mainly stabilized by a π–π interaction with the conserved Trp36 residue. The O2 and O3 atoms of XYP are stabilized by hydrogen bond interactions with the hydroxyl groups of Tyr96 and Tyr192. The conformation of the thumb domain of TsaGH11 does not play a critical role in XYP binding, and XYP binding induces a shift in the thumb domain of TsaGH11 toward the XYP molecule. A structural comparison of TsaGH11 with other GH11 xylanases revealed that the XYP molecule forms π–π stacking with the center between the phenyl and indoline ring of Trp36, whereas the XYP molecule unit from xylobiose or xylotetraose forms π–π stacking with the indoline of Trp36, which indicates that the binding modes of the substrate and XYP differ. These structural results provide a greater understanding of the recognition of XYP by the GH11 family.

Standardization of ultrasound images across various centers: M2O-DiffGAN bridging the gaps among unpaired multi-domain ultrasound images

Domain shift problem is commonplace for ultrasound image analysis due to difference imaging setting and diverse medical centers, which lead to poor generalizability of deep learning-based methods. Multi-Source Domain Transformation (MSDT) provides a promising way to tackle the performance degeneration caused by the domain shift, which is more practical and challenging compared to conventional single-source transformation tasks. An effective unsupervised domain combination strategy is highly required to handle multiple domains without annotations. Fidelity and quality of generated images are also important to ensure the accuracy of computer-aided diagnosis. However, existing MSDT approaches underperform in above two areas. In this paper, an efficient domain transformation model named M2O-DiffGAN is introduced to achieve a unified mapping from multiple unlabeled source domains to the target domain. A cycle-consistent “many-to-one” adversarial learning architecture is introduced to model various unlabeled domains jointly. A condition adversarial diffusion process is employed to generate images with high-fidelity, combining an adversarial projector to capture reverse transition probabilities over large step sizes for accelerating sampling. Considering the limited perceptual information of ultrasound images, an ultrasound-specific content loss helps to capture more perceptual features for synthesizing high-quality ultrasound images. Massive comparisons on six clinical datasets covering thyroid, carotid and breast demonstrate the superiority of the M2O-DiffGAN in the performance of bridging the domain gaps and enlarging the generalization of downstream analysis methods compared to state-of-the-art algorithms. It improves the mean MI, Bhattacharyya Coefficient, dice and IoU assessments by 0.390, 0.120, 0.245 and 0.250, presenting promising clinical applications.

Self-supervised adversarial adaptation network for breast cancer detection

Breast cancer is the most commonly diagnosed cancer worldwide, and early detection is essential for reducing mortality rates. Digital mammography is currently the best standard for early detection, as it can assist physicians in treating the disease. However, inaccurate diagnoses from mammography are common and can lead to patients undergoing unnecessary tests and treatments. To address this challenge, deep-learning techniques have shown promising results in improving the accuracy and reliability of breast cancer detection. However, existing methods face two primary challenges: the lack of the annotated data, and the inability to adapt to new data domains. In this paper, we propose SelfAdaptNet to address these issues. Specifically, SelfAdaptNet employs self-supervised learning techniques, such as Bootstrap Your Own Latent (BYOL) and Simple Framework for Learning of Visual Representations (SimCLR), to tackle the problem of limited annotated data. Additionally, the adversarial technique is used to address the problem of domain shift. By successfully reducing domain disparities, this strategy enhances the model’s adaptability and robustness across a variety of clinical scenarios. Overall, our contributions offer a more effective and flexible approach for early breast cancer detection, and experimental results demonstrate that SelfAdaptNet can produce promising results as compared with other methods.

Multi-source self-supervised domain adaptation network for VRLA battery anomaly detection of data center under non-ideal conditions

As the key component of uninterruptible power supply, the anomaly detection of valve-regulated lead-acid (VRLA) batteries is the reliable guarantee for the operation of internet data centers. Although deep neural networks (DNNs) have been investigated in battery anomaly detection, the key problem of feature learning under non-ideal conditions, i.e., data scarcity and class-imbalance, is not addressed very well. In this article, a novel DNN called multi-source self-supervised domain adaptation network (MSSSDAN) is proposed for VRLA battery anomaly detection of data center under non-ideal conditions. Firstly, multi-dimensional collaborative self-supervised learning (MDCSSL) is proposed to address the class-imbalanced problem by performing auxiliary feature learning task considering the temporal dependency of different channels. Secondly, a novel transfer learning (TL) framework, multi-source adaptive transfer learning (MSATL) is developed to solve the domain shift problem by adaptively minimizing the distribution discrepancy of different domains. Finally, the outperformance of MSSSDAN is verified on the VRLA battery dataset collected from Tencent data center. The testing results demonstrate that MSSSDAN can effectively solve the problem of battery anomaly detection under non-ideal conditions.

Unsupervised face image deblurring via disentangled representation learning

Unsupervised face image deblurring aims to restore latent clear face images with explicit structure and facial details without relying on labels. Despite the recent progress, most existing approaches ignore the domain shift issue between clear and blur domains, leading to color distortion and loss of detail in deblurred images. To tackle this issue, we propose a symmetrical unsupervised framework including a decoupling stage and a reconstruction stage. In the decoupling stage, we employ disentangled representation and adversarial domain translation to establish discriminative boundaries between content-invariant domains and feature-shift domains. Besides, a clear feature encoder for clear images is introduced to obtain a latent feature space containing more detailed clarity, providing reliable input for subsequent reconstruction. In the reconstruction stage, similar modules are adopted to ensure the integrity of the mutual mapping between clear and blurred images, while domain latent supervision loss is introduced to ensure that the decoupled features and content information from both stages belong to the same domain. Extensive experimental results on popular benchmarks validate the effectiveness of our method by surpassing existing works.

Affine Collaborative Normalization: A shortcut for adaptation in medical image analysis

The paradigm of “pretraining-then-finetuning” (PT-FT) has been extensively explored to enhance the performance of clinical applications with limited annotations. A major impediment to applying such workflow to various medical imaging modalities is the lack of parameter-free approaches boosting the transferability against notable domain shifts. The success of normalization techniques in domain adaptation provides a promising solution, while inaccessible source data in finetuning (FT) poses new challenges. In this paper, we revisit the Batch Normalization module in PT-FT and propose a unified framework for both fast transferability estimation and transferability-aware finetuning. We discovered that the vanilla FT suffers from the issue that feature patterns of corresponding channels could be misaligned across domains. Hence, we present an Affine Collaborative Normalization (AC-Norm) to dynamically recalibrate the channels in the target model according to the cross-domain channel-wise correlations without any source data and extra parameters. AC-Norm provides very competitive results compared to state-of-the-art FT methods in six classification/segmentation tasks. We also demonstrate the capability of AC-Norm in estimating the transferability of pretrained models in only single-step backpropagation. Our code is available at https://github.com/EndoluminalSurgicalVision-IMR/ACNorm.

Structure-aware feature stylization for domain generalization

Generalizing to out-of-distribution (OOD) data is a challenging task for existing deep learning approaches. This problem largely comes from the common but often incorrect assumption of statistical learning algorithms that the source and target data come from the same i.i.d. distribution. To tackle the limited variability of domains available during training, as well as domain shifts at test time, numerous approaches for domain generalization have focused on generating samples from new domains. Recent studies on this topic suggest that feature statistics from instances of different domains can be mixed to simulate synthesized images from a novel domain. While this simple idea achieves state-of-art results on various domain generalization benchmarks, it ignores structural information which is key to transferring knowledge across different domains. In this paper, we leverage the ability of humans to recognize objects using solely their structural information (prominent region contours) to design a Structural-Aware Feature Stylization method for domain generalization. Our method improves feature stylization based on mixing instance statistics by enforcing structural consistency across the different style-augmented samples. This is achieved via a multi-task learning model which classifies original and augmented images while also reconstructing their edges in a secondary task. The edge reconstruction task helps the network preserve image structure during feature stylization, while also acting as a regularizer for the classification task. Through quantitative comparisons, we verify the effectiveness of our method upon existing state-of-the-art methods on PACS, VLCS, OfficeHome, DomainNet and Digits-DG. The implementation is available at this repository.

Deep transfer learning strategy in intelligent fault diagnosis of gas turbines based on the Koopman operator

The gas turbine engine is a predominant prime mover in the transport and energy sectors, and ensuring its reliable operation holds paramount significance. While intelligent fault diagnosis (FD) approaches have seen successful advancements within the gas turbine FD landscape, many existing methods operate under the assumption of identical health states during both data collection and the FD process. Moreover, most previous studies have overlooked the diagnosis of both sensors and actuators. Another critical challenge lies in isolating simultaneous and multiple faults and providing compromising FD performance, especially in the face of continued system performance degradation. Aiming at these problems, this study develops a novel unsupervised data-driven FD strategy based on leveraging the potential of transfer learning and the Koopman operator. A deep neural network-based transfer learning framework is proposed for realizing a precise adaptive linear model called the deep transfer linear (DTL) model enabling reliable prediction of the system’s behavior in various situations and designing structured fault residuals. To this end, a deep neural network framework is used to obtain a precise Koopman model realization using the richly collected data in the source domain. Subsequently, the realized model is fine-tuned for the target domains associated with the degraded system, mitigating the adverse effects of domain shift and addressing the rich data scarcity problem in the target domain. In addition, the dedicated and generalized residual sets are designed and generated employing the geometric approach for fault isolation and a decision-making analysis is developed to diagnose simultaneous faults. The reliability of the proposed strategy is demonstrated through various experiments in the presence of noise and performance degradation, and a comparative performance analysis is conducted between the proposed strategy and another data-driven method showcasing the superiority of the proposed approach.

Meta-learning based infrared ship object detection model for generalization to unknown domains

Infrared images exhibit considerable variations in probability distributions, stemming from the utilization of distinct infrared sensors and the influence of diverse environmental conditions. The variations pose great challenges for deep learning models to detect ship objects and adapt to unseen maritime environments. To address the domain shift problem, we propose an end-to-end infrared ship object detection model based on meta-learning neural network to improve domain adaptation for target domain where data is not available at training phase. Different from existing domain generalization methods, the novelty of our model lies in the effective exploitation of meta-learning and domain adaptation, ensuring that the extracted domain-independent features are meaningful and domain-invariant at the semantic level. Firstly, a double gradient-based meta-learning algorithm is designed to solve the common optimal descent direction between different domains through two gradient updates in the inner and outer loops. The algorithm enables extraction of domain-invariant features from the pseudo-source and pseudo-target domain data. Secondly, a domain discriminator with dynamic-weighted gradient reversal layer (DWGRL) is designed to accurately classify domain-invariant features and provide additional global supervision information. Finally, a multi-scale feature aggregation method is proposed to improve the extraction of multi-scale domain-invariant features. It can effectively fuse local features at different scales and global features of targets. Extensive experimental results conducted in real nighttime water surface scenes demonstrate that the proposed model achieves very high detection accuracy on target domain data, even no target domain data was used during the training phase. Compared to the existing methods, our method not only improves the detection accuracy of infrared ships by 18%, but also exhibits the smallest standard deviation with a value of 0.93, indicating its superior generalization performance.

Decentralized federated domain generalization with cluster alignment for fault diagnosis

Fault diagnosis is important for maintaining safety in industrial scenarios. Due to the complex operating conditions, there is usually a domain shift between training (source) data and testing (target) data. Recent years have witnessed the emergence of numerous transfer learning methods dealing with the domain shift. However, existing methods often fail to deal with the situation where target data are unavailable. Moreover, the majority of these methods require aggregating data distributed across various users (nodes) for model training, raising privacy concerns. Despite existing federated learning methods can protect privacy, they mostly rely on a central server, which may lead to a single point of failure. To address the above issues, we propose a fully decentralized Federated Domain Generalization with Cluster Alignment (FDG-CA), which deals with the domain shift problem without accessing target data and eliminates the need of a central server while protecting privacy. During the training phase, the proposed FDG-CA learns domain-invariant representations by aligning clusters statistics of different source nodes through information exchange. Subsequently, during the testing phase, we propose an ensemble strategy based on a learner filter and a voting scheme to get the prediction results. Experiments demonstrate that our proposed method is superior to existing methods, achieving higher accuracy while addressing privacy concerns.

Survey of automatic plankton image recognition: challenges, existing solutions and future perspectives

Planktonic organisms including phyto-, zoo-, and mixoplankton are key components of aquatic ecosystems and respond quickly to changes in the environment, therefore their monitoring is vital to follow and understand these changes. Advances in imaging technology have enabled novel possibilities to study plankton populations, but the manual classification of images is time consuming and expert-based, making such an approach unsuitable for large-scale application and urging for automatic solutions for the analysis, especially recognizing the plankton species from images. Despite the extensive research done on automatic plankton recognition, the latest cutting-edge methods have not been widely adopted for operational use. In this paper, a comprehensive survey on existing solutions for automatic plankton recognition is presented. First, we identify the most notable challenges that make the development of plankton recognition systems difficult and restrict the deployment of these systems for operational use. Then, we provide a detailed description of solutions found in plankton recognition literature. Finally, we propose a workflow to identify the specific challenges in new datasets and the recommended approaches to address them. Many important challenges remain unsolved including the following: (1) the domain shift between the datasets hindering the development of an imaging instrument independent plankton recognition system, (2) the difficulty to identify and process the images of previously unseen classes and non-plankton particles, and (3) the uncertainty in expert annotations that affects the training of the machine learning models. To build harmonized instrument and location agnostic methods for operational purposes these challenges should be addressed in future research.