Good bets and bad bets
It’s the first day of Advent! Welcome to a series of blog posts about automated testing.
Why do we spend time on tests? This applies to both automated and manual testing.
Looked at from the point of view of the business sponsors, investing in testing is a gamble; there is an up-front cost (the developer time needed to write the tests) and an uncertain reward (stopping a future bug, or enabling future changes to happen faster). We ought to choose our test strategy to maximize our expected return on the bet.
Some tests are bad bets, because they will never catch a bug (or at least not one that justifies the effort of the test):
Sometimes the main risk on a project is in a particular area or non-functional requirement. For example, you might have a system where the performance of the code is critical; a performance regression test would be a good bet here, but that same test could be a waste of time on less important systems.
So the goal of our testing is not to have 100% coverage, but rather to attempt to save more time/cost in the long run than the cost of maintenance.