Software quality kqulitysofwarenstheproution of quaitr This book introduces a set of techniques which hold the potential for remarkable improvements in the quality of software products. Before studying these techniques,we must clarify their goals.Software quality is best described as a combination of several factors.This chapter analyzes some of these factors,shows where improvements are most sorely needed,and points to the directions where we shall be looking for solutions in the rest of our journey 1.1 EXTERNAL AND INTERNAL FACTORS We all want our software systems to be fast,reliable,easy to use,readable,modular, structured and so on.But these adjectives describe two different sorts of qualities. On one side,we are considering such qualities as speed or ease of use,whose presence or absence in a software product may be detected by its users.These properties may be called external quality factors. Under"users"we should include not only the people who actually interact with the final products,like an airline agent using a flight reservation system,but also those who purchase the software or contract out its development,like an airline executive in charge of acquiring or commissioning flight reservation systems.So a property such as the ease with which the software may be adapted to changes of specifications-defined later in this discussion as extendibility-falls into the category of external factors even though it may not be of immediate interest to such"end users"as the reservations agent. Other qualities applicable to a software product,such as being modular,or readable, are internal factors,perceptible only to computer professionals who have access to the actual software text. In the end,only external factors matter.If I use a Web browser or live near a computer-controlled nuclear plant,little do I care whether the source program is readable or modular if graphics take ages to load,or if a wrong input blows up the plant.But the key to achieving these extemal factors is in the internal ones:for the users to enjoy the visible qualities,the designers and implementers must have applied internal techniques that will ensure the hidden qualities.1 Software quality Engineering seeks quality; software engineering is the production of quality software. This book introduces a set of techniques which hold the potential for remarkable improvements in the quality of software products. Before studying these techniques, we must clarify their goals. Software quality is best described as a combination of several factors. This chapter analyzes some of these factors, shows where improvements are most sorely needed, and points to the directions where we shall be looking for solutions in the rest of our journey. 1.1 EXTERNAL AND INTERNAL FACTORS We all want our software systems to be fast, reliable, easy to use, readable, modular, structured and so on. But these adjectives describe two different sorts of qualities. On one side, we are considering such qualities as speed or ease of use, whose presence or absence in a software product may be detected by its users. These properties may be called external quality factors. Under “users” we should include not only the people who actually interact with the final products, like an airline agent using a flight reservation system, but also those who purchase the software or contract out its development, like an airline executive in charge of acquiring or commissioning flight reservation systems. So a property such as the ease with which the software may be adapted to changes of specifications — defined later in this discussion as extendibility — falls into the category of external factors even though it may not be of immediate interest to such “end users” as the reservations agent. Other qualities applicable to a software product, such as being modular, or readable, are internal factors, perceptible only to computer professionals who have access to the actual software text. In the end, only external factors matter. If I use a Web browser or live near a computer-controlled nuclear plant, little do I care whether the source program is readable or modular if graphics take ages to load, or if a wrong input blows up the plant. But the key to achieving these external factors is in the internal ones: for the users to enjoy the visible qualities, the designers and implementers must have applied internal techniques that will ensure the hidden qualities