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Therac-25 Case

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Exercises for Therac-25

Role Playing the Case

Have students read the case, including the background materials. Do not allow student to read any of the accident reports. Assign particular groups to prepare to defend the viewpoint of each of the participants in the case (AECL, FDA, Hospital, Operator). In class, give each group the description of the two Tyler incidents. Also give to them the explanation of the Tyler code and why it produced Malfunction 54. This is what each participant knew shortly after the Tyler accidents.

Allow each group 15 minutes to produce a proposal regarding what should be done. Keep this part of the assignment vague enough to allow them to propose a wide variety of remedies if they desire.

Allow each group 3 minutes to propose its remedy and each group 3 minutes to comment after hearing all the proposals.

Class discussion can initially center on which proposals are better. Use your knowledge of the case to present the Yakima accidents and ask them which of their proposals would have helped prevent that case. This will allow you to point out the larger issues involved in designing for safety: safety is a system property and not just a property of the software itself.