Variability in selecting design values in Queensland: A calibration of professional opinions

Burt Look and Natalie Campbell


Selection of design values is often subjective although statistical approaches may also be adopted. The design value is derived from the characteristic value which is a cautious estimate of the parameter. However many practitioners rely on their judgment in arriving at a characteristic value. The use of statistics provides transparency, especially when a third party is required to review the parameter selection, as often personal judgment lacks accountability. However, to be useful, a sufficient number of test results should be obtained for statistics to be used.

This paper describes a survey carried out to assess this personal judgment, and examines how the background of the engineer influences what design value is selected. The case study is for a local road with soaked California Bearing Ratio (CBR) values provided. This survey also compares the background of participants in terms of work type as well as number of years of experience to assess how we select design values. When the participants are informed of the relative cost of their decision then a comparison of the survey results (before and after) was then used to see this effect on the recommended design value.

These “professional opinion” values were compared with statistically derived results to assess our risk appetite in Queensland (as compared to elsewhere).