Software Engineering-Software Reviews


Software reviews are a "filter" for the software engineering process. That is, reviews are applied at various points during software development and serve to uncover errors and defects that can then be removed. Software reviews "purify" the software engineering activities that we have called analysis, design, and coding. Freedman and Weinberg  discuss the need for reviews this way:

Technical work needs reviewing for the same reason that pencils need erasers: To err is human. The second reason we need technical reviews is that although people are good at catching some of their own errors, large classes of errors escape the originator more easily than they escape anyone else. The review process is, therefore, the answer to the prayer of Robert Burns:

O wad some power the giftie give us
to see ourselves as other see us

A review—any review—is a way of using the diversity of a group of people to:
1. Point out needed improvements in the product of a single person or team;
2. Confirm those parts of a product in which improvement is either not desired or not needed;
3. Achieve technical work of more uniform, or at least more predictable, quality than can be achieved without reviews, in order to make technical work more manageable.

Many different types of reviews can be conducted as part of software engineering. Each has its place. An informal meeting around the coffee machine is a form of review, if technical problems are discussed. A formal presentation of software design to an audience of customers, management, and technical staff is also a form of review. we focus on the formal technical review, sometimes called a walkthrough or an inspection. A formal technical review is the most effective filter from a quality assurance standpoint. Conducted by software engineers (and others) for software engineers, the FTR is an effective means for improving software quality.

Cost Impact of Software Defects

The IEEE Standard Dictionary of Electrical and Electronics Terms (IEEE Standard 100- 1992) defines a defect as “a product anomaly.” The definition for fault in the hardware context can be found in IEEE Standard 610.12-1990:
(a) A defect in a hardware device or component; for example, a short circuit or broken wire. 
(b) An incorrect step, process, or data definition in a computer program.
 Note: This definition is used primarily by the fault tolerance discipline. In common usage, the terms "error" and "bug" are used to express this meaning. See also: data-sensitive fault; programsensitive fault; equivalent faults; fault masking; intermittent fault.

Within the context of the software process, the terms defect and fault are synonymous. Both imply a quality problem that is discovered after the software has been released to end-users (or to another activity in the software process). In earlier chapters, we used the term error to depict a quality problem that is discovered by software engineers (or others) before the software is released to the end-user (or to another activity in the software process).

The primary objective of formal technical reviews is to find errors during the process so that they do not become defects after release of the software. The obvious benefit of formal technical reviews is the early discovery of errors so that they do not propagate to the next step in the software process.

A number of industry studies (by TRW, Nippon Electric, Mitre Corp., among others) indicate that design activities introduce between 50 and 65 percent of all errors (and ultimately, all defects) during the software process. However, formal review techniques have been shown to be up to 75 percent effective in uncovering design flaws. By detecting and removing a large percentage of these errors, the review process substantially reduces the cost of subsequent steps in the development and support
phases.


Defect Amplification and Removal

A defect amplification model [IBM81] can be used to illustrate the generation and detection of errors during the preliminary design, detail design, and coding steps of the software engineering process. A box represents a software development step. During the step, errors may be inadvertently generated. Review may fail to uncover newly generated errors and errors from previous steps, resulting in some number of errors that are passed through. In some cases, errors passed through from previous steps are amplified (amplification factor, x) by current work. The box subdivisions represent each of these characteristics and the percent of efficiency for detecting errors, a function of the thoroughness of the review.
Figure given  below illustrates a hypothetical example of defect amplification for a software development process in which no reviews are conducted. Referring to the figure, each test step is assumed to uncover and correct 50 percent of all incoming errors without introducing any new errors (an optimistic assumption). Ten preliminary design defects are amplified to 94 errors before testing commences. Twelve latent errors are released to the field. It considers the same conditions except that design and code reviews are conducted as part of each development step. In this case, ten initial preliminary design errors are amplified to 24 errors before testing commences.

Only three latent errors exist. Recalling the relative costs associated with the discovery and correction of errors, overall cost (with and without review for our hypothetical example) can be established. The number of errors uncovered during each of the steps noted in figure is multiplied by the cost to remove an error (1.5 cost units for design, 6.5 cost units before test, 15 cost units during test, and 67 cost units after release). Using these data, the total cost for development and maintenance when reviews are conducted is 783 cost units. When no reviews are conducted, total cost is 2177 units—nearly three times more costly.

To conduct reviews, a software engineer must expend time and effort and the development organization must spend money. However, the results of the preceding example leave little doubt that we can pay now or pay much more later. Formal technical reviews (for design and other technical activities) provide a demonstrable cost benefit. They should be conducted.
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