The Delphi technique uses expert judgement and whilst it is sometimes helpful in estimating, is commonly used to determine the possible risk events on a project. The experts involved will generally be external to the project, possibly to the project organization; and the technique is used when embarking on a project where there is little or no experience held within the organization on projects of this particular type or area.

How to use it:

Define the problem – Identify the problem you want to work on (e.g. what are the risks?)
Give everyone the problem – Recruit people to the Delphi group – EG. the external experts. Send the problem to everyone and ask them to respond.
Collate the responses – Take all the responses that have been sent back to you and collate these into a single list or set of lists.
Give everyone the collation – Send the collated list back to everyone with the request to score each item on a given scale (typically 1-5). You may also ask them to add further items as appropriate.
Repeat as necessary – The process may now be repeated as many times as deemed appropriate to ascertain adequate risk identification or task effort/duration, when used for estimating.