## Results
### Answers to Research Questions  
The abstracts of 20 selected studies were subjected to the coding. The codes extracted from the abstracts were merged as well as relabeled to yield a meaningful classification scheme. Finally, the information extracted from the abstracts were used to populate the classification scheme to answer the formulated research questions. Apart from deriving the classification scheme, this mapping study also employs the already available classification scheme for classifying the research studies on basis of types [10] and the contributions [11]. The mapping study yielded responses to the formulated research questions as follows:  
**RQ 1**. : _What areas in requirement engineering are addressed and how many articles covers the different areas?_  
The mapping study resulted into five sub areas of requirement engineering that attracted the researchers' attention namely requirement elicitation, requirement prioritization, requirement documentation, requirement validation, and product validation. However, few studies were based on requirement engineering in general also. Since in mapping studies, different researchers could arrive at different schemes, so the authors made following assumptions while arriving at the classification scheme. * A requirement validation activity is considered as the activity undertaken to make a prototype. This prototype is used to verify the requirements and gather more information on basis of user interaction. Also, the prototype is considered as different from the minimal viable product (MVP) by the authors. * Product validation is the activity undertaken to verify if the solution delivered fulfils the basic needs of the users. In other words, experimentation undertaken by launching the MVP is termed as product validation in this paper. * The studies reporting MVP, experimentation, software evolution are categorised under the category of product validation. This assumption is held until and unless these studies focuses on individual requirement engineering activities (like documentation, prioritisation etc.) in relation to experimentation, evolution or MVP. * Few studies focused on multiple activities of requirement engineering (including generic requirement engineering). The contribution of the study is divided equally amongst the multiple focused activities (excluding the activities that are just focused minorly, for instance, activities just described in the studies). For example, a study is focusing mainly on two activities i.e., requirement engineering (complete) and requirement validation. The focus on prioritization is just at introductory levels. The contribution of the study will be counted as \(\frac{1}{2}\), individually for both majorly focused activities (sum is unity), excluding the prioritization activity (minor focus doesn't get contribution). * Studies reporting knowledge management are categorised in the requirement documentation category. This is because, knowledge management is not a requirement engineering activity, but it is a supporting activity. Table 5 shows the focus of the research studies from 2015 to 2020 (end inclusive). \begin{table}
\begin{tabular}{c c} \hline \hline
**Parent Reference Number** & **Child Reference Number** \\ \hline
[18] & [27] \\ \hline
[5] & [28] \\ \hline
[20] & [26; 29] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Relation between Parent & Child studies. As shown in Table 5, major studies are focused on generic requirement engineering (40%), followed by product validation (20%), requirement elicitation (17.5%) and requirement validation (12.5%). Requirement prioritization and documentation are the least focused activities (5% each). **RQ 2. **_How does the research addressing the different requirement engineering areas can be classified in terms of research type and contribution?_  
The research studies were classified using existing conceptual schemas i.e., on basis of types [10] and the contributions [11] (Figure 1). Table 6 gives the frequency of research classified on basis of type.