Rules of Thumb
The pyramid takes its shape from experience, which has taught us that it is better to do the bulk of our automated testing down near the bottom, where the tests are fast and cheap, than at the top where they are slow and expensive.
Not all projects have or need end-to-end UI-style tests. Some get by with just unit and integration.
That’s why whenever we go and add new tests to the system, we always start at the bottom first, and work our way up from there.
 | When adding a new test, always see if you can cover it with a unit test first. ... |
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