The Delphi method (also called Delphi study, Delphi procedure or Delphi survey) is a systematic, multi-stage survey procedure with feedback. It is intended to enable assessments of future events, technical developments, etc. 
The aim of this method is to collect forecasts and assessments of future events, trends, solutions to complex problems, to generate new ideas, or to determine opinions on an unclear issue. 
An anonymous circle of experts receives a written survey in order to provide an assessment on a specific topic. The answers are then analyzed and a new questionnaire is created based on them. The new questionnaire also gives the experts the opportunity to adjust the original answers by showing the analyzed results of the first survey.  This process is repeated as often as previously determined based on the estimated variation in the experts' assessments .
-  Linstone, H. A., & Turoff, M. (Eds.). (1975). The delphi method (pp. 3-12). Reading, MA: Addison-Wesley.
-  McMillan, S.S., King, M. & Tully, M.P. (2016). How to use the nominal group and Delphi techniques. Int J Clin Pharm 38, 655–662. https://doi.org/10.1007/s11096-016-0257-x
-  Skulmoski, G. J., Hartman, F. T., & Krahn, J. (2007). The Delphi method for graduate research. Journal of Information Technology Education: Research, 6(1), 1-21.