Chapter 26. XP Process Elements Explained
There isn't nearly enough written about techniques for writing User Stories. However, the best practices suggest that Stories must be contained on a single index card. If the organization is interested in reducing variance across teams and scaling XP across a large organization, developers should assess each Story for risk and complexity. Risk is assessed on scale of high, medium, or low. Complexity is assessed on a 3-point scale.
Assessing User Stories
Assessing risk and complexity allows estimating of Stories to become more scientific and predictable. The estimated LOE for Stories, in each of the 9 possible weightings of risk versus complexity, should converge within a single organization over time. ...