[josephg]

When I’ve gone full TDD on projects, I’m always surprised to find there’s a power law distribution or something on failing tests. Most of the tests I write never catch a bug in the life of the software. Something like 90% of the value of a test suite could be achieved with only about 10% of the tests. Of course, the trick is figuring out which tests are going to repeatedly fail ahead of time. But there’s something interesting down this rabbit hole.

I wonder if it would be worth collecting per-test stats through the life of a project to explore this.

[lmm]

You'd get a lot of the value by just being willing to delete tests once it's clear they're not serving any purpose. Sadly a lot of people would rather see a high test coverage percentage than have an effective test suite.

[nitrogen]

What is the value of deleting a test once it's written, though?

[lmm]

Tests impose a maintenance burden like any other lines of code. Just making it harder to navigate to relevant code (including useful tests) is a significant cost.

[ornornor]

You’ve already paid the cost for writing the test, might as well keep it.

If a test fails when modifying the codebase or adding a feature or updating dependencies, it still serves a purpose in my opinion even if fixing it takes some time.

Having the test no matter how trivial also helps boost confidence that a new change didn’t break anything no matter how trivial.

But tests should be refactored once in a while, and if a thing is complicated to setup for testing, either the thing to test should be refactored before shipping and/or the developer working on that thing should also ship a factory and helper functions for testing that thing.

[lmm]

> If a test fails when modifying the codebase or adding a feature or updating dependencies, it still serves a purpose in my opinion even if fixing it takes some time.

> Having the test no matter how trivial also helps boost confidence that a new change didn’t break anything no matter how trivial.

If it catches an error in your change, that's a benefit. If it delays a correct change with false positives, that's a cost. (And if a passing test "boosts confidence" but the change is actually broken, that's another, more subtle cost). Often the cost is larger than the benefit.

> But tests should be refactored once in a while, and if a thing is complicated to setup for testing, either the thing to test should be refactored before shipping and/or the developer working on that thing should also ship a factory and helper functions for testing that thing.

That's all valid assuming the tests are providing some value. But just like with any code, the first question should be whether it's serving a useful purpose at all - if not, then deleting is cheaper and more effective than any amount of careful refactoring.