When human error is involved in the error chain of accidents in complex systems, the causes of the errors are very seldom the result of: 1. a random slip, 2. one inadvertent oversight, 3. a single unintended action, 4. one mis-perceived event, 5. simple mal-performance, of a complex action, or 6. a poor training program for the human operators. Invariably, the cause of the break down is in a very poor conscious choice by someone from the operator back through system designer, the supervision, management and leadership through the entire organization. Usually the operator bears the burden of the blame and is either rebuked, retrained or replaced. In systems such as commercial aviation, complex manufacturing systems, powerplants, process control systems, information-processing systems and communications networks, the replacement or retraining of individuals or even classes of individuals usually does not result in any long-term improvement of the safety or effectiveness of the system. What is needed is a system that identifies the reasons why the operators made the errors. Further a system is needed that can recommend what can be done to improve the future performance within the system. The professional performance analysis system (PPAS) has been developed and applied to more than 50 major aircraft accidents in the past 30 years. The PPAS is a direct outgrowth of the human performance analysis system developed by Robert Mager over 45 years ago. The PPAS system is applied after a complete and unbiased definition and description of the events of the accident or incident has been developed by the teams of accident investigation and accident reconstruction professionals. The PPAS then uses a systematic protocol and algorithm to determine the reasons as to why the humans committed the errors or why they performed at subnormal performance levels. This process is based on quantitative behavioral science principles and findings that have been demonstrated valid for many generations. The PPAS looks at five attributes of human performance to identify the factors that can be changed to improve performance in the future. The result is a series of objective definitions of changes that have been demonstrated to be the attributes that influence human performance. The PPAS provides a tool to the analyst, who lacks sophistication or experience in quantitative behavioral sciences, to arrive at recommendations that are based on validated human performance principles. (C) 2004 Elsevier B.V. All riahts reserved.