## 3 Methodology
### Three-Round Delphi Questionnaire  
After identifying the potential metrics from the literature review and in-depth interviews, a three-round online Delphi technique was employed to establish consensus [46]. The main objective of the three rounds was to reach a consensus by reducing variance in responses, and the second purpose was to improve precision [47]. During the Delphi study, participants were allowed to suggest additional statements that had not been included in the previous step. The participants were informed that there would be three rounds of questionnaires. Survey participants were identified using purposive sampling [48]. The Delphi survey included selected mining experts, consultants, and lecturers. The participants were chosen from a pool of mining experts who were familiar with mining transportation as well as senior consultants and lecturers from reputable organizations. We invited all the mining experts who were involved in the prior in-depth interviews to take part in the Delphi survey. In addition, we invited eight additional mining experts to take part in the Delphi survey; fourteen participants were invited in total. We began the survey by circulating the questionnaire via electronic mail to all participants. Then we collected their responses and returned the questionnaire to them for other rounds. Feedback is an essential component of the Delphi procedure [49]. Key information from the preview survey was used to design the subsequent Delphi survey questions. In the later rounds, the participants were able to view the group's responses. The data were stored on the computer with password protection and could only be accessed by the interviewer. The Delphi survey participants are described in Table 4. Mining experts were required to have at least ten years of work experience in mining operations. The lecturer was required \begin{table}
\begin{tabular}{c c c} \hline \hline
**Participant** & **Position** & **Work Experience** \\ \hline Participant \#1 & Managing Director & 23 years \\ Participant \#2 & Operation Manager & 22 years \\ Participant \#3 & Improvement Manager & 12 years \\ Participant \#4 & Operation Superintendent & 20 years \\ Participant \#5 & Production Planning Superintendentent & 10 years \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Profiles of the mining experts. to have research or teaching experience in total productive maintenance (TPM) and overall equipment effectiveness (OEE) and be familiar with the coal mining industry. Consultants were required to have experience in implementing TPM and OEE and have experience in handling clients in the mining industry. Questionnaire preparation included pilot testing (pre-testing) of the questionnaire, which was conducted by an assessment group [50]. The pre-testing was aimed at checking instruction and question clarity, completion time, and question redundancy [51; 52]. We used the feedback from the pilot test to modify, add, or delete ambiguous questions to improve the convergence of the questionnaire [53]. The questionnaire was sent to panel experts, but because of time limitations and the pandemic situation, the survey was conducted by email. The first-round questionnaire included three parts, and the first part collected generic information such as the company's field of operation, position, and experts' years of experience and email addresses. The second part presented all the potential metrics from the previous process (literature review and in-depth interview) according to the 5-point Likert scale of 1 (not agree) to 5 (extremely agree). We designed a survey questionnaire in which experts were asked to judge the level of significance of each metric. These questions aimed to enable the panel experts to give their opinion of the importance of each metric based on their understanding and experience. The third part was an open-ended question, where the participants were allowed to suggest additional statements about potential metrics that had not been included in the list.