Safety factors Judgement

Weil, C.S. 1972. Statistics versus safety factors and scientific judgement in the evaluation for man. Toxicol. Appl. Pharmacol. 21 454-463. 

On a world-wide basis, the safety factor has been set on an ad hoc basis as a matter of expert judgement on the basis of all the available relevant evidence. In the United States, it has usually been set at 100, and this is also possibly the most common factor world-wide. 

The confidence of risk estimation depends on a combination of the uncertainty associated with any threshold value used and the error of measurements used to test whether that threshold value is exceeded. In most cases, neither have well-defined confidence intervals. Factors that can be used to assist judgements about the robustness of risk estimation include the extent to which safety factors have been included in the derivation of threshold values and the assumed errors of measurement from sampling and testing. 

In some cases the designer assumes in the safety analyses the specification value of the leakage rate to be increased by a certain factor chosen by good judgement. If the leakage rate is 0.2 per cent, in the safety analyses a value of 1 per cent is sometimes used. This is a matter of opinion, however it is certainly better than directly using the specification value without the support of previous applicable experience. 

The selection of a contractor has to be a balanced judgement with a number of factors taken into account. Fortunately, a contractor who works well and meets the client s requirements in terms of the quality and timeliness of the work is likely also to have a better than average health and safety performance. Cost, of course, will have to be part of the judgement but may not provide any indication of which contractor is likely to give the best performance in health and safety terms. In deciding which contractor should be chosen for a task, the following should be considered ... 

Keywords safety, safety assessment, safety case, confidence argument, evidence, evidential reasoning, human factors, expert judgement, uncertainty, confidence. 

Expert judgement plays a vital role in this process. However, the developer or the assessor can never be 100% certain that all hazards were mitigated. Furthermore, uncertainties might exist from secondary issues, such as who created the safety case, who was responsible for generating the evidence, what types of tools and techniques were used, etc. These confidence factors often tend to be implicit considerations in the development and assessment of safety cases. 

The method for classifying the safety significance of a structure, system or component shall primarily be based on deterministic methods, complemented where appropriate by probabilistic methods and engineering judgement, with account taken of factors such as ... 

The Court of Appeal reduced the fine but upheld the Crown Court s earlier view on costs. In giving judgement, it was said that the required standard of care does not depend on the size of the employer s business or on its financial position. The Court of Appeal also commented that the low level of risk involved in the task which gave rise to a fatal electrocution was not a mitigating factor. It accepted that the company s means and previous good record could be taken into account. The degree of risk, the extent and duration of the breach of the law, and the fact that active steps were taken to improve safety following the incident were also taken into account. 

Experiments have shown that judgements of the potential of accidents made by safety experts are reasonably reliable. At the workplaces, the supervisors may be assigned the task to make such judgements. Experience shows that a number of subjective factors influence the supervisor s assessments. There are biases in the direction of lower-risk... 

Expert judgements are in many cases the only practical way to establish basic causal factors. This condition has some important implications. The person with direct responsibility for safety at the site of the accident, i.e. the supervisor, should not make the judgements alone. A consensus process is recommended, where persons with direct knowledge of the circumstances and persons with different responsibilities and perspectives take part in the investigation into causal factors. 

The ISA shall ensure that their judgements regarding safety are not influeneed by inappropriate pressures or other factors. 

---