Value of Early Fit Testing and the Concept-of-Fit (COF)

Fit tests are structured, systematic methods to determine the range of fit in a size and compare design alternatives against a defined set of criteria, referred to as the Concept-of-Fit (COF). They turn subjective preference into a transparent, auditable process that reduces risk, improves design, and increases stakeholder confidence. Early fit studies are cheap insurance against expensive mistakes. They maximize your ability to influence the project outcome with minimal investment, leading to better-performing, lower-risk, and more cost-effective products.
Some people have argued that fit testing does not work, because the subjects can’t tell us what fits and what does not in a consistent manner. It is true that subjects are not good at determining fit. That is why we don’t let them define it. We might ask subjects to rate the discomfort or the look, but we don’t let them decide if it fits. We develop, document, and agree upon how the product should fit, what constitutes a good fit versus a poor fit, and methods for assessing or measuring it. This is called the COF. (Robinette et al 2025) The COF also defines which conflicting aspects have priority or are essential. For example, if a coverall is comfortable and functions well but makes the person look sloppy or fat is that acceptable? If a head wearable is very comfortable, but it interferes with other equipment how much interference is acceptable? Finally, it builds stakeholder consensus with an objective, documented rationale.

For new products it is important to begin fit testing early in the product development process for several reasons:

1.Design Freedom is Highest Early On

Early phases have the broadest set of possible alternatives. As the project progresses, decisions "freeze" the baseline. Changing direction later becomes exponentially more difficult and expensive. Early tests explore the full trade space before committing to a specific path.

2.Cost of Changes Rises Dramatically Later

This is one of the most cited principles in systems engineering (often shown as the "cost of change curve"). Late fit testing can result in the need to rework, retrofit, or to lose sales.  Locking in suboptimal designs or sizes can become painful (or impossible) to fix later.

3.Requirements Are Still Flexible

Early fit tests help balance and refine requirements themselves. They reveal which requirements drive cost, risk, or complexity the most, allowing intelligent trades (e.g., relaxing a performance requirement slightly to reduce number of sizes needed).

4.Resource Efficiency

Early tests prevent wasting effort on design concepts or sizes that would ultimately fail. They focus subsequent engineering work on the most promising path.

5.Size Optimization

With early fit testing we determine the range-of-fit in at least one size, and who we are fitting versus who we are not fitting. This permits us to optimize our sizing for our targeted population, remove unnecessary sizes, reduce redundancy, include sizes where the population is concentrated, and produce the right amount of each size.

More information on fit testing and the COF can be found in  Robinette, K.M., Veitch, D., Alemany, S., & Bredenkamp, K. (2025). Product Fit and Sizing: Sustainable Product Evaluation, Engineering, and Design (1st ed.). CRC Press. https://www.routledge.com/Product-Fit-and-Sizing-Sustainable-Product-Evaluation-Engineering-and-Design/Robinette-Veitch-Alemany-Bredenkamp/p/book/9781032491189

If you would like someone to help, Anthrotech provides this type of consulting service.