Many causes of the software crisis can be traced to mythology based on

Concept: a software myth is a widely believed but false statement about software and about the process used to build it. Pressman classifies these myths by who holds them — managers, customers, and practitioners — and shows that each class sets an expectation the real process cannot meet. The software crisis is the name given to the resulting pattern of missed schedules, cost overruns, and defective, hard-to-maintain products. Tracing a cause of the crisis therefore means identifying whose false beliefs produced the unmet expectation.

Application: each of the three groups Pressman names contributes its own distinct failure mode to that pattern.

• Managers believe a shelf of standards or a purchase of state-of-the-art tools by itself guarantees quality, and that adding programmers to a late project recovers it — beliefs that produce schedule slippage and budget overrun (Brooks's law states the opposite: adding people to a late project makes it later).

• Customers believe a general statement of objectives is enough to begin coding, and that requirements can be changed late at little cost because software is flexible — beliefs that produce requirements churn and repeated rework.

• Practitioners believe the job is finished once the program is written and running, and that quality cannot be judged before the program executes — beliefs that produce latent defects and code that is expensive to maintain.

Cross-check: no single one of the three groups accounts for the whole pattern. Eliminate the management myths and the rework driven by customer myths still overruns the budget; eliminate the customer myths and the practitioner myths still leave defects in delivered code. Because each group independently generates crisis symptoms, the mythology behind the crisis spans all three classes rather than any one of them.

Hence the causes are traced to management myths, customer myths and practitioner myths taken together — the combined choice, "All of the above".