This refers to the number of expert panelists to be included in the
study (Polit and Beck 2008). There are no clear guidelines suggesting
the numbers to be included in studies applying the Delphi survey
because the sample is purposively selected and it depends on the
problem being investigated. Some studies have used 15 participants
whilst others have used 60 [35]. Needham and de Loë in 1990 suggest a
sample size of a minimum of 10 (a smaller size does not generate
enough ideas) and a maximum of 50 participants (a larger sample
results in cost inefficiencies related to time, product and the iteration
process). Whilst, DeVilliers, De Villiers and Kent in 2005 define
sample size depending on whether it is homogenous or heterogeneous
and suggest the following numbers: if they are from the same
discipline (15 – 30) or from differing ones (5–10) per professional
group. Delphi survey studies do not call for a representativeness of the
sample in terms of statistical purposes; therefore, sample size
principles differ from those in other surveys [34].