This is a “rule of thumb” that has been borne out over time. (The ratio may vary a bit from case to case, like when the experience levels are different across the roles.) This ratio combines with the specialization of principle #6 to form the strong basis for the Cascadeeffect covered in the last principle below.
I use the most common ‘role titles’ of analyst, designer, developer, and tester for the remainder of this book. Sometimes these roles have different names, and there are many other roles in IT overall, but these four are the most recognizable to anyone doing primarily project work. (And the Project Manager makes five! Can’t forget them…)