## 3 Research Method
### Paper Selection: Inclusion/Exclusion Criteria  
The studies selection process included mainly two tasks: proper definition of the include and exclude criteria and the application of these defined criteria to choose the most relevant primary published papers [42; 43].  
As we know that the inclusion and exclusion criteria are totally two opposite actions, the authors decided to concentrate the efforts on exclusion, by properly defining a set of criteria that is both objective and subjective. The former does not cause any type of threat to the validity, and hence, its application is quite simple. Applying these first exclusion criteria, particularly those concerning language and duplicity, allowed the authors to eliminate irrelevant content quickly. The following list contains the objective exclusion criteria applied to the retrieved published research studies:  
* EC1: Papers written in other languages except English;
* EC2: Short published research studies (less than four pages in length);
* EC3: Research studies not published in some peer-reviewed venues;
* EC4: Not a primary research study (secondary and tertiary studies, if any, were considered in the Related Work section of this study);
* EC5: Grey literature (books, slide presentations, forewords, talks, etc.);
* EC6: PhD or Master Theses, under the assumption that relevant publications resulting from them were already published as research papers on peer-reviewed venues;
* EC7: Duplicate reports of the same study (consider only the most recent one),  
The subjective criteria are the trickiest to handle. They possibly can bring bias into the research study, and a predefined protocol must be used to lessen this threat as much as is viable. Contrastingly, the application of these criteria produces the greatest decrease in the number of research studies to deem as relevant. The authors applied mainly two exclusion criteria stated below:  
* Not Focus: Not relevant to the application of controlled vocabularies in software development;
* Out of Scope: Not relevant to any of the requirements engineering phase of software development lifecycle,  
Any research study not excluded by the aforementioned criteria was finally comprised in the set of _selected primary research papers_.  
A top-down approach was followed to apply these criteria. At first, some particular metadata information such as the title, abstract and keywords was taken into consideration. If this information were not sufficient to exclude the research study at hand, then the authors comprehensively reviewed the full text, specifically the Introduction (problems and contributions of the study), Results and Conclusions sections.  
To perform the papers selection process, the papers were equally distributed among authors. A review meeting was held to ensure the smoothness of the process, no relevant paper is overlooked and no irrelevant paper is included.  
To deal with disagreements, we applied the inclusive criteria proposed in [34] and detailed Table 7. We excluded a paper only when both reviewers agreed (category \"F\") or considered the paper as borderline (category \"E\").  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**SB Iteration** & **Seeds** & **Citations** \\ \hline First & 70 & 1176 \\ \hline Second & 14 & 162 \\ \hline Third & 5 & 99 \\ \hline Fourth & 1 & 9 \\ \hline Total= & 90 & 1446 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Snowballing search results.