The most important requirements for the use of Delphi are the need for experts' judgment, group consensus to achieve the results, anonymity in data collection, a complex, multidimensional, and interdisciplinary problem, lack of consensus and imperfect knowledge, experienced and capable experts, dispersion of experts, no time limitation, and lack of cost-effective method [39] [6] [40] [28] [7] . In qualitative research that is mainly based on the individuals' judgments and opinions, there is a prerequisite for using the Delphi method. If these studies are based on the expert opinions, then the use of inferential statistical techniques such as mean tests will not be justified. Since either experts are not so much that can be accessed wide samples of them or the time and cost to access them are difficult. In addition, to review the experts' opinions this proverb is truer: "you may know by a handful the whole sack". Therefore, the two types of qualitative research should be distinguished to use the Delphi technique. Delphi technique can be used for qualitative research that is exploratory and identifying the nature and fundamental elements of a phenomenon is a basis for the study. Consider the problem of measuring customer satisfaction. If you were to examine the customer satisfaction, statistical sampling and inferential statistical methods would be used. However, if you decide to measure the customer satisfaction, then experts' opinions and the Delphi technique can be used. One of main application of Delphi technique is screening the items in operations research problems and of multi-criteria decision-making (MCDM) techniques.