When is increased testing effort justifiable?

Is a high testing effort affordable and justifiable? Jerry Weinberg’s response to this question is: “Compared with what?” [DeMarco 93]. This response points out the risks of using a faulty software system. Risk is calculated from the likelihood of a certain situation arising and the expected costs when it does. Potential faults that are not discovered during testing can later generate significant costs.

Example: The cost of failure

In March 2016, a concatenation of software faults destroyed the space telescope Hitomi, which was built at a cost of several hundred million dollars. The satellite’s software wrongly assumed that it was rotating too slowly and tried to compensate using countermeasures. The signals from the redundant control systems were then wrongly interpreted and the speed of rotation increased continuously until the centrifugal force became too much and Hitomi disintegrated (from [URL: Error Costs]).

In 2018 and 2019 two Boeing 737 MAX 8 airplanes crashed due to design flaws in the airplane’s MCAS flight control software [URL: MAX-groundings]. Here too, the software—misdirected by incorrect sensor information—generated fatal countermeasures.

Testing effort has to remain in reasonable proportion to the results testing can achieve. “Testing makes economic sense as long as the cost of finding and remedying faults is lower than the costs produced by the corresponding failure occurring when the system is live.”⁵ [Pol 00]. Reasonable testing effort therefore always depends on the degree of risk involved in failure and an evaluation of the danger this incurs. The price of the destroyed space telescope Hitomi could have paid for an awful lot of testing.