Doctor: In this room, we have the Therac-25. To tell
us all about this wonderful device is our senior technician, Fred.
Fred: The Therac-25 is a miracle of modern medicine.
With it, people who would have been in pain for the rest of their
lives are magically healed!
Kyle: Does anything ever go wrong?
Fred: Something, went wrong once, but that's all in the
past. The machine never did it again, and the company who makes the
machine says everything is fine with it. The accident was just a
freak of nature. The Therac-25 comes from a long line of very
reliable machines. Parts of its software come from the Therac-20
and Therac-6, and since they've never had problems, the software in the
Therac-25 is 100% reliable.
Narrator: We now know that there were problems in the software
for the Therac-20 and Therac-6. The previous Theracs
had hardware interlocks that prevented the machine from performing certain
actions if something went wrong. When something did go wrong, the
machine stopped, so technicians never reported anything wrong with it.
Because of that safety record, the designers of the Therac-25 believed
the software to be very reliable. The interlocks that were in the
previous versions of the Therac were replaced by software in the new
version. Error messages appeared so often on the Therac-25, technicians
believed it was just a part of the deal. Most of the errors never