This represents the generally accepted qualification route through a validation exercise and has done so for many years now, but back in May 1987, when validation was in its infancy and the FDA issued its Guideline on General Principles of Process Validation, things were different:

DQ was not a regulatory requirement, and isn’t even today (quite unbelievably).

IQ was to ensure that “process equipment and ancillary systems are capable of consistently operating within established limits and tolerances”.  Not any more, it isn’t.

OQ (whether Operation or Operational) did not exist.  It still doesn’t as far as the guide is concerned, but they certainly expect it to be done.

PQ did exist, but only as part of PPQ, which stood for either Process Performance Qualification or Product Performance Qualification.

Since 1987, all attempts by FDA to update the guide have been thwarted, deemed too prescriptive and definitive by an industry that, while demanding more detailed guidance, insisted on the leeway for individual interpretation.