The Richter Scale of Culture was a method of describing, quantifying and comparing the development and achievement of numerous disparate cultures encountered on different worlds.
Developed by Dr. Alonzo Richter in the 2200s, it took into account information across a broad spectrum of scientific disciplines. A Richter Scale descriptor could include up to three preface letters, 21 explanatory qualification digits, five exception letters, and twelve philosophical-comparison pointers. For most non-experts, the preface letter provided a generalized description of a cultures level, ranging from "AA" (no tool use) to "Q" (no technology beyond current Federation theoretical science). Beyond "Q" was "XX", describing a culture which appeared beyond understanding.
Alonzo Richter continued to refine his scale throughout the early and mid 23rd century, to the point of such complexity that only about 1000 specialist were able to formulate a rating for any given culture. The majority of these individuals were employed by Starfleet's First Contact Office. (TOS novel: Prime Directive)
Federation experts rated the Organians at level "D minus" on the Richter Scale, presumably corresponding to an agrarian culture with hand or animal powered machinery, as this race initially presented themselves. (TOS episode: "Errand of Mercy")