This note addresses the four variables: time, scope, resources and quality. The fourth variable quality has been described as “strange” and non-linear. I think the problem is that it isn’t a variable at all. Rather, quality is a measurable output value, not a controllable input variable. Risk is the fourth input variable that can be controlled by the customer.
When time, scope, and resources are defined, the risk of failure is constrained. And, that’s the best you can do. Plenty of projects have succeeded with reasonably high quality despite shoestring budgets and unreasonable scope expectations. They are clearly in the minority, but you don’t guarantee failure by unreasonably constraining the first three variables. You increase the risk, and being the customer, that’s your prerogative. If you want to decrease risk, you reduce scope. Increasing time decreases implementation risk, and increases time-to-market risk. Increasing or decreasing resources is more complicated, but changing resources affects risk whereas it may have no affect on quality.
The customer models risk to gauge a project’s chance of success. The risk model is usually implicit, but it’s as real as the implicit models programmers use to estimate stories and tasks. One of the factors in a risk model is historical quality. For most customers, historical quality boils down to: team A produces better software than team B, so let’s go with team A. As XPers, we can help our customers do better.
Many people measure quality in terms of MTBF and MTTR. (This is not internal quality, which I discuss briefly below in .) However, the meaning of F (failure) and R (repair) is often fuzzy and incomplete. With XP stories, we have an accurate and complete measure.
I consider a failure to be any event that results in a story–be it fixing a defect, adding documentation, or enhancing the system. Failure is failing to meet an expectation. (This may happen multiple times before the story is implemented, and I’m not exactly sure how to integrate that data, but counting stories is a reasonable starting starting point. Another measure of quality is to count defects instead of failures.) Time to repair is the elapsed time it takes to deliver a story’s implementation. These numbers are readily available for an XP project, and define quality for its entire life.
With this definition of quality, the customer has practical data about an XP project’s quality at any point in time. That is, the customer has an actual number instead of a “feeling” which can be used to refine her risk model for future releases and projects. Quality can’t be guaranteed, but historical quality data allow the customer to better understand and control the relationship between the four variables: time, scope, resources, and risk.
 MTBF is Mean Time Between Failures
 MTTR is Mean Time To Repair
 Internal quality is just increased scope. Ron Jeffries says this at ^http://c2.com/cgi/wiki?FourVariables. Ideally, all work is on story cards, and that should include the time to refactor, i.e. increase internal quality. If you don’t like that, reduce resources by X% to account for the implicit work involved in refactoring. Either way, internal quality is a subjective measure that probably influences external quality (MTBF and MTTR) but cannot guarantee a particular value of MTBF and/or MTTR. If it could, we found the silver bullet to ensuring external quality for every single project, not just improving the average.