mvogl is correct in that the PIE chart shown on the link he provided was used in the development of 217Plus. It is the same percentage/distribution information referred to by dnicholls.
However, it represents an industry “average”. As dnicholls pointed out, an organization’s experience/processes could lead to a different distribution.
Both the “Handbook of 217Plus Reliability Prediction Models” and the “Reliability Modeling – The RIAC Guide to Reliability Prediction, Assessment and Estimation” texts provide a scoring methodology (via a questionnaire related to one’s processes) where one can ‘score’ their processes, and using simple formulas (also provided n the texts) , allows the derivation of the relative effects (percentage-wise) that their processes have on reliability.
The RIAC also offer a spreadsheet version of that same questionnaire “217Plus Process Grade Scoring.xls” which helps calculate the percentages. Although its name implies that it is meant for use with 217Plus (and it is), it could be used as a stand-alone assessment tool.