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Exercises for Therac-25

Software Safety Myths

In her book Safeware: System Safety and Computers (p. 26) Nancy Leveson lists seven myths regarding the safety of software.

  1. The cost of computers is lower than that of analog or electromechanical devices.
  2. Software is easy to change.
  3. Computers provide greater reliability than the devices they replace.
  4. Increasing software reliability will increase safety.
  5. Testing software and formal verification of software can remove all the errors.
  6. Reusing software increases safety.
  7. Computer reduce risk over mechanical systems.

After having the class explore the Therac 25 case, ask students to evaluate the truth of each of these statements as they pertain to the case. This can be done either as part of a homework assignment, with class discussion after papers are turned in, or as a class discussion followed by individual papers. Alternatively, you might combine these two approaches and have students turn in a paper and then revise it (or write a short postscript) based on class discussion.


Leveson, N. G. (1995). Safeware: System safety and computers. New York: Addison Wesley.