One of the things that I'm asked to do fairly regularly is define the roles for an agile team, be it for Scrum, XP or some other flavour of agile. I usually end up doing this based on required skills, but my first impulse is to refuse, because I don't believe that a self-organising team actually has roles and I think that defined roles limit both the individual and the team. Let me examine that point before I explain how I do go about defining roles and why.
There is one role on a self-organising team - team member. The responsibility of that role is to bring all their skills to bear on the team's problems and organise their contributions to maximise the team's effectiveness.
When something needs to be done on an agile team everyone on the team should be able to see it, and in many cases it will be obvious which individual or individuals are the best suited to getting it done. The simplest approach is to allocate tasks based on skills, but the team might decide to use other criteria as well - they may need to grow their capacity in some area and decide to give the task to someone with less experience to give that person some practice; they may have limited capacity and be forced to give the task to someone who needs to learn the required skill, usually with mentoring from someone on the team. There will also be team-specific and project-specific reasons for not allocating tasks based simply on skill set. The important thing is that the reasons are well-articulated, and that the team member accepts responsibility for the task rather than having it arbitrarily assigned to them.
Although multi-skilled teams are normal and effective, provided the team is dedicated to achieving a result, has sufficient time and is reasonably self-aware you can imagine any arbitrary skills starting point. We could take a team that consisted entirely of testers and give them some work to do. The first tasks they encountered would require some analysis, so someone from the team would need to do this. They wouldn't complete the analysis as quickly or to the same quality as someone with the relevant experience, and they might not enjoy it as much. They might also need to get outside expertise (training and/or mentoring), but they would get through the work eventually. The team might find someone who wanted to grow into this role and make them the primary analyst, or they might decide that no one enjoys it and the role should be rotated through the team (this is probably less efficient, but it explicitly ranks satisfaction and retention ahead of efficiency). The same pattern can be repeated with development and anything else the team needs to get done.
Don't get me wrong - this is clearly a slow path to success and I wouldn't recommend it, but it is a valid approach. It's a contrast to the behaviour I see more often, which is "I can't do X, because I'm a Y". "Can't" is definitely the wrong word - finding the right word will determine how you respond to this assertion. If the person is really saying "I don't like doing X" then keep the assignment short and try to find some other way to make it attractive. If they're saying "I don't know how to do X", then they need training, guidance and support. If they're saying " I won't do X" then they probably shouldn't be part of the team.
That's why I don't like roles - why do I provide descriptions of roles and responsibilities? It's because I would rather start with a team with diverse skills than spend a lot of time developing the same skills, and most organisations understand that as a request for roles. I also appreciate that most people have gravitated towards a role that suits their preferences and skills, and asking people to move into other areas requires both time and motivation that we usually don't have a the beginning of project.
My hope is that companies that go to the trouble of assembling multi-discipline teams will also recognise the benefits of keeping those teams together for long periods of time. Time is the essential ingredient to building role-independent teams - it helps build trust between the team members, it lets people experiment and learn, and it gives the team opportunities to get feedback on different approaches to task assignment.
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