# Smart Parking: A Literature Review from the Technological Perspective
## 2 Research Methodology
### Perform Review Phase  
During this phase, specific strings, which are defined in the previous phase, were searched into every digital research repository. To present the results a table was built. This table contains the name of the digital repository in its header (one for each one), the search string used, and the results obtained for every search string. The results obtained after performing such queries are shown in Table 1.  
From the search results, only recent solutions were considered (after 2013) because technology becomes obsolete after 5 years, on average, as stated in [(24)]. First of all, with the aid of the result exporting feature provided by each scientific database repository, CSV was the format selected to export search results sets, as SpringerLink uses it by default. CSV documents containing search results were obtained. These files were later loaded in a tool to preview information such as title, authors, abstract and years. The tool called Zotero was used to execute such procedure. Zotero is an open source tool developed by a nonprofit entity. It was selected because it is able to process and deterge records in CSV and BIBTEX format. Likewise, it can generate \".bib\" or \".sqllite\" files, which are both supported by Mendeley. This tool allowed to conduct a first review of the found articles. Some of them had incomplete information, so they had to be filled manually. The most important field checked was DOI, as it contains their main identification. For the next step, a \".bib\" file with information of all the reviewed articles was generated, and a private group for collaboration was created in Mendeley where these records were imported and stored. Then, PDF documents of the selected articles were retrieved and uploaded inside this tool. As a result, a subset of previously retrieved papers was obtained. Finally, a protocol for discarding articles was followed. This protocol was executed manually, and it consisted on reviewing the title and the abstract of the papers which were collected from different digital repositories, to find keywords and brief details that might help to answer the research questions. If there were no terms or information related to the topic, such a paper is discarded. Then, the remaining papers were analyzed to identify if they were focused on the research topic. Finally, a second round or the process was performed to obtain a considerable number of papers for this research. A representation of the discarding protocol followed is shown in Figure 2. \begin{table}
\begin{tabular}{l c c c} \hline \hline \multicolumn{2}{c}{**IEEE Xplore Digital Library**} & \multicolumn{2}{c}{**ScienceDirect**} \\ \hline
**Search String** & **Results** & **Search String** & **Results** \\ \hline (((((smart parking) & & smart parking & 10 \\ AND solution) AND sensors) & 49 & solution and sensor & 10 \\ NOT algorithms) & (((smart parking) & & \\ AND systems) AND sensors) & 172 & smart parking & 13 \\ NOT algorithms) & & systems and sensors & 13 \\ \hline \multicolumn{2}{c}{**ACM Digital Library**} & \multicolumn{2}{c}{**Springer**} \\ \hline
**Search String** & **Results** & **Search String** & **Results** \\ \hline +smart +parking & & smart AND parking & \\ +solutions + sensors & 11 & AND solutions AND sensors & 137 \\ \(-\)algorithms & & AND NOT (algorithms) & \\ +smart +parking & & smart AND parking & \\ +sensors \(-\)algorithms & 39 & AND solutions AND system & 53 \\  & & AND NOT (algorithms) & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: IEEE Xplore Digital Library, ScienceDirect, ACM Digital Library, and Springer query results. During the first iteration, the first phase ended with a list of 484 articles, which included not relevant documents for this study. To filter relevant documents, titles and abstracts were read and analyzed. Filtering was performed by tagging these documents with a \"YES\" if it was useful and \"NO\" for irrelevant. During the first round, 414 papers were discarded, as they do not contain pertinent information.