The Art of Software Testing, Second Edition

2.3. Software Testing Principles

Continuing with the major premise of this chapter, that the most important considerations in software testing are issues of psychology, we can identify a set of vital testing principles or guidelines. Most of these principles may seem obvious, yet they are all too often overlooked. Table 2.1 summarizes these important principles, and each is discussed in more detail in the paragraphs that follow.

Principle 1: A necessary part of a test case is a definition of the expected output or result.

This obvious principle is one of the most frequent mistakes in program testing. Again, it is something that is based on human psychology. If the expected result of a test case has not been predefined, chances are that a plausible, but erroneous, result will be interpreted as a correct result because of the phenomenon of "the eye seeing what it wants to see." In other words, in spite of the proper destructive definition of testing, there is still a subconscious desire to see the correct result. One way of combating this is to encourage a detailed examination of all output by precisely spelling out, in advance, the expected output of the program. Therefore, a test case must consist of two components:

1. A description of the input data to the program.

2. A precise description of the correct output of the program for that set of input data.

Table 2.1. Vital Program Testing Guidelines

Principle Number	Principle
1	A necessary part of a test case is a definition of the expected ...