What we did do was spend the last five years hammering out our proprietary methodology, based upon our collective Price Waterhouse Urwick and Andersen Consulting/Accenture experiences.
We noticed that despite EVM's precision, we still did not always get the desired result. Projects still went over budget with alarming frequency (although we rarely went over the scheduled delivery date). Analysing this showed us that in many cases a task that had a 'generous' estimate given to it was not completed under budget, but in fact took up the whole of the time allocated.
Further analysis showed that this was a combination of people fitting other unplanned (and often nonchargeable) tasks into the time required for their project tasks, and people simply coasting or 'spinning their wheels' unnecessarily (e.g. over-engineering the solution because they had more time than required for the original design).
Of course we had yet to perfect this method, but we had already recognised that:
Where we still needed to improve was in the way we calculated the size of and tracked the use of buffers. However, it is interesting to see how our proprietary methodology mirrored recent changes in PMBOK (i.e. the introduction of EVM in the 2000 edition), and even included ideas from new methods such as Critical Chain Project Management without any of us attaining formal project management certifications.
* You could say we did follow PMBOK, because the term is defined inclusively:
“The Project Management Body Of Knowledge (PMBOK) is an inclusive term that describes the sum of knowledge within the profession of project management. As with other professions such as law, medicine and accounting, the body of knowledge rests with the practitioners and academics that apply and advance it.”
PMBOK Guide - 2000 Edition