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How many tests of which type should be in the system.
In the real world this often does not hold up
While Unit test are meant to be the backbone of your application testing
Acceptance tests often give a more conclusive picture of the application as it usually exercises the whole stack.
But keep in mind that
- Unit tests are faster,
- Acceptance tests are coming to be slower.
- this is downside of Acceptance tests.
- why acceptance is on the top?
  - is that are way more costly to run, than the unit tests.
As a unit test exercise a small piece of code as seen before

Reference