Requirement ysis - How to ensure correctness & completeness ?

Every requirement should have a corresponding validation that goes with it as part of the contract. That validation is something that comes in a format that software test procedures can be directly derived from (i.e. specific line items with a quantitative measure to them). Judging completeness should come down to a simple checkbox on a spreadsheet. Judging what is sufficient ultimately comes down to the customer and the contract. You absolutely need experienced technical professionals in order to ist in the requirements process. Not having this is like trying to plan a skyscr without a foreman and an architect...just office managers. There are different kinds of requirements for different contexts. The larger the software, the more technical issues there will be that will inevitably need to be verified with the customer. Having someone with domain experience up front will minimize the number of re-considerations that are necessary throughout the process and keep you from becoming victims of the software cycle.