What a TRL number does and does not tell you

A Technology Readiness Level (TRL) is a single number. It is useful for a rough ranking of development stage, and easy to misread. This guide explains what it covers and what it leaves out.

What the scale is

NASA describes TRL as "a type of measurement system used to assess the maturity level of a particular technology". The scale runs from TRL 1 to TRL 9, and each level has its own description (NASA TRL definitions).

The levels, as summarised on the NASA page:

• TRL 1: basic research is starting, and its results are feeding future research and development.
• TRL 2: basic principles have been studied, and practical applications can be applied to those initial findings. The technology is very speculative, with little to no experimental proof of concept.
• TRL 3: active research and design begin, usually with analytical and laboratory studies, often including a proof-of-concept model.
• TRL 4: multiple components are tested together.
• TRL 5: more rigorous testing of a "breadboard" technology, ideally in near-realistic simulated environments.
• TRL 6: a fully functional prototype or representational model exists.
• TRL 7: the working model or prototype is demonstrated in a space environment.
• TRL 8: the technology has been tested and "flight qualified", ready to be integrated into an existing system.
• TRL 9: the technology has been "flight proven" during a successful mission (NASA TRL definitions).

What a TRL number tells you

• A rough position on a development path, from early research (low numbers) to proven in operation (high numbers).
• That the level reflects how far testing has progressed. For example, TRL 5 means more rigorous testing than TRL 4, and TRL 6 means a fully functional prototype exists (NASA TRL definitions).

Points to check when a TRL number is quoted

The points below are not statements from NASA. They are things a reader should check when a TRL number appears in a headline or article.

• Check how well the technology works. A level describes how far development has reached, not the result.
• Check whether the result holds for other people, places or applications. A prototype tested in one setting may not have been tested in another.
• Check whether cost, manufacturing scale, supply chains, regulatory approval or market demand are covered by the claim.
• Check whether safety or regulatory approval has been shown separately. The NASA level descriptions do not mention it (NASA TRL definitions).
• Check whether the wording has been adapted. The NASA definitions refer to a space environment and "flight qualified" (NASA TRL definitions). Applying the same numbers to a battery, a drug or a food product means adapting the wording, and the adaptation should be stated.
• Check which application the number refers to. A technology may have several applications at different stages.

How Vrulum uses it

Vrulum does not show a bare TRL for every technology. It uses broad stages (Theory, Lab demonstration, Research and prototypes, Pilot and early commercial, Commercial, Not assessed). It applies a NASA-style TRL (1 to 9) only when a level can be justified for a specific application and the reasoning is published. Otherwise the TRL is shown as "not assessed" (Vrulum methodology).

Vrulum's readiness rating is separate from its ratings of evidence quality and momentum, and the three are not combined into one score (Vrulum methodology).

Checklist before quoting a TRL

• Which application is the TRL about?
• Is the reasoning for the level published, and where?
• Is the level based on a published experiment, or on a company claim? (See the claim labels in the Vrulum methodology.)
• Is the wording of the NASA level adapted for this field, and has that adaptation been stated?
• Does the sentence say what the level does not cover, such as cost, safety approval or results in other settings?
• Is the source dated, and has the technology moved on since?