While a system traditionally is modeled by structural decomposition into structural elements, the dynamic behaviour of systems and their actors is modelled by decomposition of the behavioral flow into events, acts, decisions, and errors. Such decomposition is the basis for identification of activity elements in terms of 'tasks' and task elements in terms of 'acts.' The problem is, that all work situations leave many degrees of freedom for choice by the actors, even when the objectives of work are fulfilled.

To complete a description of a task as being a sequence of acts, these degrees of freedom must be resolved by assuming additional performance criteria that appear to be 'rational' to a task analyst or instructor. They cannot, however, foresee all local contingencies of the future work context. In particular, a rule or instruction is often designed separately for a particular task in isolation whereas, in the actual situation, several tasks are active in a time sharing mode that poses additional constraints on the procedure to use.

These constraints are often not known by designers and work planners. In consequence, rules, laws, and instructions practically speaking are never followed to the letter. Strikes by civil servants take the shape of "working-according-to-rules." Even for highly constrained task situations such as nuclear power operation, modification of instructions is repeatedly found and the operators' violations of rules appear to be quite rational, given the actual work load and timing constraints. One implication in the present context is that following an accident it will be easy to find someone involved in the dynamic flow of events that has violated a formal rule just by following established practice. He or she is therefore likely to be exposed to punishment.

Consequently, accidents are typically judged to be caused by 'human error' on part of a train driver, a pilot, or a process operator. A task description or an instruction is an unreliable model for judging behavior during actual work, as found in a dynamic society

 

Jens Rasmussen & Inge Svedung, Proactive Risk Management in a Dynamic Society.  Risk & Environmental Department, Swedish Rescue Services Agency, Karlstad First edition, 2000