## 4 Results and Discussion
### Results for Non-IID Data Studies  
This subsection illustrates the systematic mapping results of works that have proposed techniques for solving the non-IID data problem. A total of 93 papers passed the selection criteria. Figure 7 shows the non-IID types that have been simulated in the non-IID studies; as shown in the figure, label distribution skews such as quantity label imbalance and distribution label imbalance were widely addressed. The citation of these studies is presented in Appendix A Table A1. Figure 8 shows the top ten techniques most utilized in these studies, representing the central focus of this study in guiding FL users, researchers, and providers when implementing an FL approach. The figure indicates that enhancing the aggregation method and clustering are the most widely used techniques to overcome the non-IID data problem, appearing in 18% (17 studies), and 16% (15 studies) of the selected works. Personalized federated learning was used in 13% (12 studies) of the works to design a personalized model incorporating client data. An adaptive approach was utilized in 11% (10 studies) of the works. Client selection and data sharing were used in 9% (eight studies) of the works. Regularization was used in 5% (five studies), while knowledge distillation was used in 4% (four studies) of the works. Hierarchical and hierarchical clustering techniques were used in 3% (three studies) of the works to overcome the non-IID data problem. The citation of these studies is presented in Appendix A Table A2. \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Publication Venue** & **Type** & **No.** & **\%** \\ \hline
2022 IEEE/CVF Conference on Computer Vision and Pattern Recognition (CVPR) & Conference & 3 & 3.23\% \\ IEEE Transactions on Parallel and Distributed Systems & Journal & 3 & 3.23\% \\
2022 IEEE International Conference on Big Data (Big Data) & Conference & 2 & 2.15\% \\
2022 IEEE International Conference on Data Mining (ICDM) & Conference & 2 & 2.15\% \\ ICC 2020—2020 IEEE International Conference on Communications (ICC) & Conference & 2 & 2.15\% \\
2021 International Joint Conference on Neural Networks (IJCNN) & Conference & 2 & 2.15\% \\
2021 IEEE 23 rd Int Conf on High Performance Computing \& Communications; 7 th & & \\ Int Conf on Data Science \& Systems; 19 th Int Conf on Smart City; 7 th Int Conf on & & \\ Dependability in Sensor, Cloud \& Big Data Systems \& Application & 2 & 2.15\% \\ (HPCC/DSS/SmartCity/DependedSys) & & & \\ KDD '21: proceedings of the 27 th ACM SIGKDD conference on knowledge & & & \\ discovery \& data mining & Conference & 2 & 2.15\% \\ Machine learning and knowledge discovery in databases & Conference & 2 & 2.15\% \\ Computer Vision—ECCV 2022 & Conference & 2 & 2.15\% \\ IEEE Transactions on Wireless Communications & Journal & 2 & 2.15\% \\ IEEE Transactions on Network Science and Engineering & Journal & 2 & 2.15\% \\ Future generation computer systems-the international journal of eScience & Journal & 2 & 2.15\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Publication venues with more than one selected study on solving the non-IID data problem. \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Publication Venue** & **Type** & **No.** & **\%** \\ \hline IEEE Internet of Things Journal & Journal & 7 & 9.46\% \\ IEEE Transactions on Wireless Communications & Journal & 3 & 4.05\% \\
2021 17 th International Conference on Mobility, Sensing and Networking (MSN) & Conference & 2 & 2.70\% \\
2021 IEEE 41 st International Conference on Distributed Computing & Conference & 2 & 2.70\% \\ Systems (ICDCS) & Conference & 2 & 2.70\% \\
2021 IEEE International Conference on Communications Workshops & Conference & 2 & 2.70\% \\
2022 IEEE Globecom Workshops (GC Wkshps) & Workshop & 2 & 2.70\% \\ GLOBECOM 2022—2022 IEEE Global Communications Conference & Conference & 2 & 2.70\% \\ IEEE Transactions on Network Science and Engineering & Journal & 2 & 2.70\% \\ \hline \hline \end{tabular}
\end{table}