Professor O’Neill, from the Australian National University, in a report for the Civil Aviation Authority says, “Validation is generally considered an indispensable part of model development and it would be unwise to rely on outputs that have not been subject to extensive testing and confirmation.  Professor O’Neill was highly critical of methodology which is based on interviews, providing the subjective views of stakeholders.  Professor O’Neill’s comment is as follows.  “These subjective judgments can be very inaccurate because reality can be counter-intuitive in actual practice.  If the key input assumptions and relationships are highly subjective and inaccurate, then the modelling results will also be highly subjective and inaccurate”