Essentially, an accurate mechanism does not exist for identifying the number of individuals or the number of panels for inclusion in any individual study [41]. Although there are some disagreements about the composition and panel size of Delphi technique, a dominant pattern can be detected. It has been recommended that the panel size may vary according to the topics covered, the nature of different viewpoints included, and the time and money available [18] and it is also suggested to use a combination of individuals with multiple specialties and heterogeneous groups better than the homogeneous groups [1] [26]. Hsu and Sandford (2007) claimed that, ‗‗Delphi subjects should be highly trained and competent within the specialized area of knowledge related to the target issue‘‘ (p.4) [42]. Hogarth (1978) argued that between six and twelve members are ideal for Delphi technique and according to Clayton (1997), if a mixture of experts with different specialties is used, between five and ten members are sufficient (p. 5) [26]. While some Delphi studies considered fewer than 10 members in their panels [43] [44], other studies included more than 100 participants [45] [46]. Delphi panel is done with the participation of individuals who have the knowledge and expertise of the study subject. These people are known as the Delphi panel. One of the most important phases of Delphi technique is selecting eligible members for the Delphi panel because the validity of the results depends on the competence and knowledge of panel members [1]. In previous studies, Delphi has not recommended a specific method for sampling. Snowball technique can be used for sampling. The snowball sampling is one of the most common approaches of sequential sampling. This type of sampling is a non-probability method, which is also randomly selected. This method is suitable when the members of a group or community cannot be easily identified. In this method, the researcher first identifies some people and after receiving the information, he/she wants them to introduce other people [47]. This method can be also used to identify the experts in a particular field [48].