Agile scope completion techniques

One of the questions I've received in the past about agile techniques is how to ensure you've captured enough detail about your requirements in order to proceed without missing major scope elements.

Whether you are using story cards, features or other techniques to capture your requirements, you need to answer this question: "How do I know when I've done enough requirements gathering?" In waterfall this is ‘easy’ – gather all the detail and sign-off (ok – I’m simplifying). In agile, we depend on features or stories, but many are concerned that major scope elements will be left out which will either cause many items to grow exponentially in size or that feature X is really feature X, Y and Z. For example, when the registration screen has 50 fields instead of the 10-15 that we might have assumed, but didn’t write down. It is hard to understand how this can be done in 1 or 2 days using feature or story cards that contain only one line of description, a few lines of acceptance and a few assumptions.

Three things for you to consider to help you solve this dilemna:

1. In waterfall techniques, although we hold some comfort in our massive requirements documents, we know from experience that even then things will change and things will be missed.

2. My teams estimate using planning poker with the full team including the client and we have found this has helped to uncover hidden or unknown scope. We discuss each item together before estimating and talk about the number of screens, inputs, outputs, services etc involved. This discussion itself often uncovers additional scope, but so does the estimating that follows each discussion. For example, when most of us say ‘2’ and one person says ‘8’, the person who said ‘8’ enlightens the team on the complex caching required to meet the performance requirements listed as an Acceptance test. This is especially important if your client is the one with the highest estimate. Don't ignore it.

3. Lastly, I attended a virtual class on agile estimating that suggested another technique. For every feature or story, categorize the requirements certainty as high, medium and low. Keep challenging your client until the requirements certainty on each story is 'low'.

I'd be interested in other techniques you may be using to keep the initial requirements gathering phase light weight, yet complete. I think as an industry we are getting better at embracing the changes that are inevitable on all projects, but our clients still require us to have a good understanding of the known scope and the resulting estimate before starting the project.