When caught by a fire or other urgent situations it is useful to have emergency equipment on hand and know how to use it. The same goes for Project Exception Processes, if something untoward happens then that is not the best time to be creating new processes to deal with the event and explaining how to use them. Emotions are high, people respond to bad news differently, and it is better to practice an agreed to procedure than figure out new rules.
Project tolerances and exception plans provide an agreed to emergency plan for when bad stuff happens to good projects. They act as guardrails to help prevent us going off track and provide a mutually understood and agreed to resolution process. So, just as during an emergency is not the best time to collaborate on improvising a rope ladder, nor is during a major project scope change the best time to define a resolution process between project stakeholders.
We will look at the two components (Tolerances and Exception Plans) individually and then examine how they work together. Project tolerances are the guardrails, the upper and lower boundaries the project stakeholders are willing to tolerate for a given project metric. Another way to think of it is how much slack rope we have as a project team to do our own thing (or hang our selves with). Tolerances can be set on a variety of metrics and the degree of variation will depend upon the individual risk tolerances of the collective stakeholders. Some projects might be very time critical, others more concerned with budget, or user satisfaction.