## 5 Discussion and Conclusions
Other possible research gaps that deserve further investigation include other metrics, such as statistical measures, since these automated performance metrics produce sets of numerical values, and these data require the application of further statistical analysis; however, it is debatable which statistical model is best fit to describe these data. The number of objectives: We believe in advancing the current practice of defining the objective functions in SBSE will eventually reveal new research gaps. However, there are increasing interests in the community in re-formulating MoOPs with more than three objectives. We found not all the research studies define a new problem(s), some studies apply the existed problems [102, 122, 125]. This depends on the objective of the paper, some papers intend to formulate new problems while others only propose a new algorithm or compare existed algorithms by either applying existed formulated problems or considering new problem formulation. However, this does not indicate the practitioners are relaying the existed formulated problems, since the majority of them are formulating new problems with several objective functions, while we have seen studies employing a different number of objectives in a single study [75, 76, 86, 91, 95, 96, 102, 108, 109, 117, 122, 123, 138, 146, 147]. Such practice of formulating a limited number of objectives shows the practitioners are either facing difficulties in re-formulating more objective functions that normally need a mathematical definition or defining a small number of objectives that are less expensive and easy to perform. It is worth mentioning that the community is lacking theoretical studies or discussions. Although, traditionally SBSE community re-formulated a single objective problem, and currently dominated by two to three objectives, however, this indicates, the opportunities of exploring a wide range of objectives are open issues. Software engineering application areas: We observed that some of the applied areas in SE are less explored, such as requirement, management, and verification, while some areas are highly explored, such as software testing and software design. It is worth noting, that the less dominated areas indicate there are limited problems to solve in that area while the dominant areas are considered to have more diversity of problems to solve. It is also interesting to address why some software engineering areas are less applied compared to others. Some of the factors that can be linked to this include: Some areas in software engineering disciplines are characterized to be expensive in terms of cost and time, such areas include software testing. However, decision-makers might prefer solving such constraints to have the best alternative solutions. Software testing was also considered one of those SBSE practitioners previously applying, however, over the years, the discussion was growing significantly while finding new research gaps become easy, and interest in responding to such future works is another contributing factor. Another fact is that software testing is easy to automate, thus, such automated problems are easy to measure, and such measurements are used to guide the fitness functions. Although SBSE pioneers argue that the software engineering field is rich in metric, however, this makes many areas in software engineering subfields to become fit for re-formulating as a multi-objective problem and applying search-based methods. However, it is required a future investigation and finding if software testing and software design have more metrics compared to other subfields such as management, requirement, and verification. Besides, it is recommended to explore more software engineering fields that are least applied and re-formulate their problem (e.g., formal methods). [PERSON] and [PERSON] conceived the question questions, conducted review procedure, synthesize data and wrote the paper. [PERSON], [PERSON], [PERSON], and [PERSON] contributed in reviewing method and editing the paper.