1.3 Summary

• The problem domain for software engineering is industrial-strength software. This software is meant to solve some problem of some set of users, and is expected to be of high quality.
• In this problem domain, cost, schedule, and quality are basic driving forces. Hence, methods and tools that will be used for solving problems in this domain must ensure high productivity and high quality.
• Productivity is measured as amount of output per unit of input resource. In software, output can be measured in terms of lines of code delivered, and as human time is the main resource, input can be measured as person-months. Productivity can therefore be measured as lines of code delivered per person-month.
• Software quality has many attributes which include functionality, reliability, usability, efficiency, maintainability, and portability. Reliability is often considered as the main quality attribute, and as unreliability in software is due to defects in the software, quality can be characterized by number of defects per thousand lines of code.
• The problems in this domain often tend to be very large and where the needs of the customers change fast. Hence the techniques used for developing industrial-strength software should be such that they are capable of building large software systems, and have the capability to handle changes.

1.2 Scale and Change

Though cost, schedule, and quality are the main driving forces for a project in our problem domain (of industry strength software), there are some other characteristics of the problem domain that also influence the solution approaches employed. We focus on two such characteristics—scale and change.

Most industrial-strength software systems tend to be large and complex, requiring tens of thousands of lines of code. Sizes of some of the well-known software products are given in Table 1.1. As can be expected, development of a large system requires a different set of methods compared to developing a small system, as the methods that are used for developing small systems often do not scale up to large systems. An example will illustrate this point. Consider the problem of counting people in a room versus taking a census of a country. Both are essentially counting problems. But the methods used for counting people in a room will just not work when  taking a census. A different set of methods will have to be used for conducting a census, and the census problem will require considerably more management, organization, and validation, in addition to counting.

1.1 Cost, Schedule, and Quality

Though the need for high quality distinguishes industrial strength software from others, cost and schedule are other major driving forces for such software. In the industrial-strength software domain, there are three basic forces at play—cost, schedule, and quality. The software should be produced at reasonable cost, in a reasonable time, and should be of good quality. These three parameters often drive and define a software project.

Industrial-strength software is very expensive primarily due to the fact that software development is extremely labor-intensive. To get an idea of the costs involved, let us consider the current state of practice in the industry. Lines of code (LOC) or thousands of lines of code (KLOC) delivered is by far the most commonly used measure of software size in the industry. As the main cost of producing software is the manpower employed, the cost of developing software is generally measured in terms of person-months of effort spent in development. And productivity is frequently measured in the industry in terms of LOC (or KLOC) per person-month.

1. The Software Problem

Ask any student who has had some programming experience the following question: You are given a problem for which you have to build a software system that most students feel will be approximately 10,000 lines of (say C or Java) code. If you are working full time on it, how long will it take you to build this system? 

The answer of students is generally 1 to 3 months. And, given the programming expertise of the students, there is a good chance that they will be able to build the software and demo it to the professor within 2 months. With 2 months as the completion time, the productivity of the student will be 5000 lines of code (LOC) per person-month.