8.1 Introduction

Some of the most complex systems Man has built are in software. In recent years many hardware functions have been replaced by software and thus software has become a critical part of many systems. Starting from fighter aircraft to household electronic items software plays a major role. Typical examples from the most common applications include the software controlled fuel injection system of the automobile [whose performance is vastly superior to the best carburetor] and the automobile braking system incorporating anti-skid and traction control functions. Software is the core element of today's automobile and medical industries. A growing proportion of the systems operate in real time. The operational effects of failure are large and critical. For example a breakdown of airline reservations, train reservations, banking and many other services can bring the day to day activities to a stand still. The failure of a software controlled braking system in an automobile can be disastrous. For systems such as air traffic control, space shuttle, fighter aircraft and automated missiles, the software makes a very crucial contribution to the success of the mission. Software cannot be seen or touched, but it is essential to applications such as those described above. It is necessary that the reliability of software be measured and evaluated, as it is in hardware. It is also essential to understand how Six Sigma concepts can be used in software development...