## 4 Research Implications and Limitations  
When performing automatic searches in digital libraries, one of the main objectives is to ensure the completeness of the selected studies. As mentioned earlier, a comprehensive search was performed using synonym terms related to the subject matter of the search. This helped to ensure that the data returned by the search expression had several relevant studies. However, the number of papers initially returned could be a problem but in the research development phase, the application of exclusion criteria at 2 different stages allowed studies that were not related to the research objectives to be removed. This systematic literature review has two main threats to validity--possible bias in the selection of studies and possible bias in data extraction where, because they are considerably subjective, researcher's bias may affect the results of this review. To enhance bias reduction in study selection, the review protocol was developed and validated as follows: initially the protocol was defined by a researcher and validated by another two researchers with extensive experience and practice in the field of software engineering and systematic literature review. After protocol completion it was strictly followed. To minimize the effect of possible bias by researchers on study selection, the selection process was conducted in 3 phases (see Section 2.4) to reduce the chances of exclusion of relevant studies. One researcher performed the study selection process and the two other researchers examined all included and excluded studies (inclusion and exclusion reasons were recorded and disagreements were resolved through discussions). Still in order to reduce bias in the selection of studies, only automatic searches using keywords in digital libraries were performed, where the search terms were iteratively improved based on evaluation searches and were carefully tested before executing the review. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Evidence Level**} & \multicolumn{2}{c}{**Context**} \\ \cline{2-3}  & **Academic** & **Industrial** \\ \hline
0: Without evidence & [S2], [S12], [S16], [S21], [S47], [S56], & \\  & [S60], [S62], [S66], [S67] & X \\ \hline
**1**: Demonstration or usage & [S4], [S8], [S17], [S18], [S19], [S24], & \\ example & [S29], [S30], [S51], [S53], [S55] & [S25] \\ \hline
**2**: Expert Notes & X & X \\ \hline
**3**: Laboratory experiment & [S3], [S5], [S7], [S9], [S11], [S34], [S35], & \\  & [S39], [S41], [S42], [S43], [S44], [S45], & \\  & [S46], [S48], [S49], [S52], [S54], [S57], & \\  & [S58], [S59], [S64], [S65] & \\ \hline
**4**: Empirical Investigation & X & [S1], [S6], [S13], [S14], [S28], [S31], [S32], \\  & X & [S36], [S37], [S38], [S40], [S50], [S61], \\  & & [S63], [S68], [S70], [S71] \\ \hline
**5**: Strict analysis & X & [S15], [S22], [S23], [S69] \\ \hline \hline \end{tabular}
\end{table} 
Table 12: Evidence and context. To reduce the threat of data extraction inaccuracies, a data extraction form has been created (see Table 4) to consistently extract and analyze the data needed to answer the research questions of this systematic literature review. To enable bias reduction, the data extraction process was conducted by two researchers who performed the extraction and verification of the selected works (all disagreements were resolved through discussions) and the results were validated by a third researcher. In the data extraction process, both researchers performed the complete reading of the selected studies answering the research questions (RQ1-RQ5) and the quality criteria (Q1-Q7) according to Table 4. All extracted information was recorded and a comparative analysis of the information extracted from each study was performed. Similar observations and conclusions were unified with appropriate additions and, for disagreements, they were resolved through discussions. At the end of the process, the third researcher validated the results by acting as the final decision-maker for discussions where no agreement decision was made.