The Delphi method uses iterative (e-)mailed questionnaires in successive rounds. Because there is no interaction between the respondent and the researcher, the formulation of the questions has to be clear, and definitions should be given where necessary.
The questionnaire of the first round encompasses open-ended questions, to identify items to include in the second round.
Next rounds could be exclusively qualitative or composed of closed questions with scales (from totally agree to totally disagree, i.e. from 1 to 9), or combining both qualitative and quantitative questions. They present a synthesis of the results issued from the previous round.
In the case of closed questions, agreement is usually summarized by using the median and consensus assessed by presenting interquartile ranges for continuous numerical scales97. Graphical presentations of the results are welcomed.
In KCE reports the questionnaires used in each round are presented in appendices.