How do you control software quality

I am thinking of making a video about software quality control. But wanted to get some background information.

3 Questions:

1) what bug/defect removal methods do you use – unit testing/code review/test driven development/use case reviews/code walk though – if so what

2) do you monitor defect density at all – if so how

3) do you know the ROI of your defect removal methods – what did you learn