## 2 Review Methodology
### Search Strategy
#### 2.2.2 Selection Criteria  
The entire procedure of selection criteria was divided into two phases. Phase 1, termed as the collection and analysis phase, comprised article collection, removing duplicates, and applying inclusion and exclusion criteria, as shown in Figure 2. Phase 2, termed as the synthesis phase, forwarded the refined articles to proceed further with quality analysis to refine further and narrow down the articles to analyze the methods and find results for the defined five research questions.  
Figure 2: Phase 1: collection and analysis.  
The set of articles collected as a corpus based on the SQs (SQ1-SQ3), counting to 2239 articles, included some of the inclusion and exclusion criteria except that could not be applied at this stage. The collected articles were refined further to remove duplicates and proceed with the reapplication of missed out inclusion or exclusion criteria, if any.  
Inclusion criteria:  
* Using ML to analyze the academic performance;
* Using ML to preprocess the modeling data;
* Comparative assessment of various ML methods and their results obtained;
* Journal versions--for duplicate articles, the recently published article is considered;
* Articles including online education assessment;
* Academic performance and recommendation systems.  
Exclusion Criteria:  
* Review articles;
* Book chapters;
* Factor analysis;
* Articles not written in the English language.  
Step 1: Extract all the articles from the six data sources with the predefined criteria for inclusion and exclusion.  
Using the four SQs as defined, a corpus of articles was amassed. A total of 2329 articles were collected based on the defined SQs as shown in Table 1. From this basic search retrieval, the duplicate articles were removed.  
Step 2: Remove duplicates.  
Duplicates that are obtained using search query. Since the articles chosen might find citations in several sources, this step was carried out. After the elimination of the duplicates, a total of 1593 articles for further processing were selected. For duplication of the article title from multiple databases, the database that had the article in its creamy layer, the top 50% of the total article is retained and the article in other citing database sources is eliminated.  
Step 3: Re-apply inclusion and exclusion criteria if needed.  
After the application of the same to the Title, Abstract, and Keywords, the resultant set comprised 353 articles.  
Step 4: Manual refinement of corpus.  
After manual refinement by analyzing each title, the final obtained result set mentioned as the selected dataset was 80 articles. Firstly, manual refinement was carried out by eliminating the articles that had a similar combination of title and method that seemed to be repeated. Secondly, in some cases, even if there was a match of keywords applied in the search query abstract of some articles, it did not reflect the necessary information of the review considered. The main source of manual refinement is the abstract of the article. Only the recent and complete study articles were considered.  
Step 5: The selected articles were then evaluated using the quality assessment as discussed in the next section to finalize with 56 articles.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**SQ** & **SQ1** & **SQ2** & **SQ3** & **Database-Wise Count** \\ \hline Google Scholar & 70 & 465 & 13 & 548 \\ \hline WoS & 171 & 24 & 0 & 195 \\ \hline Scopus & 3 & 170 & 1 & 369 \\ \hline ScienceDirect & 8 & 41 & 1 & 50 \\ \hline SpringerLink & 1043 & 0 & 100 & 1143 \\ \hline IEEE Explore & 0 & 24 & 0 & 24 \\ \hline Query-Wise Count & 1295 & 724 & 115 & 2329 \\ \hline \hline \end{tabular}
\end{table}
Table 1: A Summary of the collected articles.