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Subtle Variables, Big Disasters

Let’s consider some famous software failures to see how important, and how difficult, it can be to identify subtle variables.

The Therac-25 Case

In the late 1980s, a number of cancer patients treated with the Therac-25 radiation therapy machine sickened after their treatment. Some died. The cause of death was radiation poisoning.

Investigation revealed that under certain circumstances the machine delivered an overdose of radiation rather than the safe medical dosage that the doctor prescribed. The investigation team concluded that the root cause was not a single failure but rather a complex set of interacting variables. Although hardware safety deficiencies contributed to the malfunction, the software had serious ...