Without testing, how can you be sure?

There's a lot of questions about "Is testing really needed". By the way, the correct answer is Yes. It seems that during development most developers say they'll develop the tests for their own equipment. This will ensure the unit won't be tested well.

Developers don't necessarily design for testability. Quite often they design connector's that can't be mated to easily, circuit cards that are embedded into the system to where they can't easily be tested, or circuit cards that just hard to deal with.

Designers know what parts of their design is weak and typically test in a way that their circuit will pass. Not consciously, but more of a sub-conscious because they don't want to see their baby fail. Or testing to what is designed and not in a way to truly verify the design.

I say testers are needed and need to be brought in early if they are to be able to help. Also, since quite often management don't see proper testing as a necessity, the test budget and schedule is shorted or cut to the bone. This will hamper the the testing effort and typically cause shotty testing of a unit.

If shotty or only partial testing is desired before sending a unit out to a customer, bring testers in late, or not at all. If you want to ensure the customer is satisfied, bring in the testers early and let them make sure a good job is done.