(Source: XP-mailinglist; thoughtful post by Ken Schwaber) Scrum is purely a project/product management process that can be applied to software projects, hardware projects, marketing projects, and any mix of the above. It does not contain engineering practices or any specific work practices. It is instead a way to maximize the ROI of work.

People who use Scrum in software development environment often adopt one or more of the XP practices to improve the engineering practices of the develpment organization. Scrum calls for an increment of potentially shippable product functionality every iteration. This means a fully cleaned up, refactored, tested, documented ready to go increment. Many organizations are incapable of this. It is easy to bring in XP practices because they are excellent and both Scrum and XP are pretty radically agile.

Project managers using Scrum are called ScrumMasters to differentiate the type of work they do ... they facilitate, manage the process, and optimize the team's productivity. They don't tell the team what to do, nor do they set of pairs of programmers, or parse out user stories. During the iteration, the team is entirely self- organizing ... whether it is doing software development or anything else related to the increment of functionality they are building.

XP seems to focus on team productivity... doing something the right way and as productively as possible. Scrum does this some, but instead focuses more on doing the right thing, getting ROI from building the 20% of the functionality that is necessary to get the value and maybe not building the rest.

We've implemented Scrum without telling the customer, users, or stakeholders. We've done this in one day. We seduce them into iterative, incremental development where they collaborate with us on what to do next. XP seems to require a steeper implementation curve with less acceptance from the users and customers. Scrum can't keep the customers away.

Estimating is a subtle diffrence that points out the XP and Scrum dividing point. XP works hard to estimate very finely defined stories, and to measure and improve these estimates. Scrum keeps the requirements more broad, more in general user terms that are analyzed during the iteration. Estimating isn't as important. The team does what it can, and gets better and better at figuring out how much it can do each iteration as it learns each other, the business domain, and the technology - iteration by iteration. We care more about delivering business value that having defensible estimates, which become meaningless in a collaborative setting.

Scrum also has a formal methodology that lays out how to scale Scrum to any sort of project with any number of people in any number of locations ... all based on the optimized 7 person team. This is an important mangement requirement, but not so important to an engineering discipline like XP.

I feel we are blessed to have such compatible practices and processes to apply to our software engineering projects.