Large organisations often ousource software projects. For this purpose they create tender documents, which are used by software companies to make a bid.
Tender documents must specify requirements so that interested companies can estimate the effort needed. What is the best format for these requirements ? Use cases, user stories, traditional functional requirements, high level features ?
If you write detailed uses cases, you will need a lot of work up-front, and you have the risk of missing details that will be charged additionally once they are found by the contractor.
If you write high level features, the estimates done by the software companies will not be accurate, and you have the risk the company will underestimate a complicated software project.
What are your thoughts and experiences on this issue ?
Thank you in advance.