In the microeconomics books you can meet the term: opportunity cost or alternative cost.
When me make a decision, we discard the the alternatives. With the opportunity cost we can give a weight of the lost possibilities, in other words the benefit we would have gotten if we had chosen to do something else, the roads not taken. It can happen that we don’t even consider in other options other than what we are doing, because we are busy concentrating on the obvious.
Tl;dr: every opportunity has a cost.
We can apply this concept the world of testing as well.
The eternal truth is we cannot test everything. That is why there are decisions to be made.
Do we spend time on performing the right testing activity? Could we do a better or different approach to gain confidence?
We might spend most of the time on polishing a new feature, covering as many aspects of testing as possible so that the new feature is thought to be rock solid leaving other test activities behind.
We might spend most of the time on fixing automated tests. Automation is a good investment if we do it in a way that it can give us valuable data in a short manner or in areas what are not easy to cover otherwise. But does it worth to maintain an automated test case that has no – or has a relatively small value?
The value of an automated test case is a question of two things. The impact if something bad is happening, and the likelihood of not finding the issue during day-to-day testing activities without the automated test case.
Retrospectively things becomes obvious. It is easy to be smart when the consequences are clear.
Applying the measurement of opportunity cost we could be able to make better decisions in the given circumstances.