Setting Quality Specifications

A quality specification defines the tests and acceptance criteria a batch must meet before it can be released - and setting it well is a balancing act. Limits that are too tight risk rejecting batches that are perfectly fit for purpose; limits that are too loose risk letting substandard product through. This guide explains how to approach setting specifications for a product made by a contract manufacturer.

What a Specification Covers

A quality specification typically lists the tests to be performed on a batch, the analytical method used for each test, and the acceptance criteria - the numerical range or qualitative standard the result must fall within for the batch to pass. For a chemical product, this often includes properties such as purity, physical characteristics, and any performance-related properties specific to the product's function.

Balancing Tightness Against Manufacturability

Setting specifications too tightly risks batches being rejected even when they are genuinely fit for purpose, creating cost and delay without a real quality benefit. Setting them too loosely risks allowing product through that doesn't reliably perform as intended. The right balance depends on which properties are genuinely critical to how the product performs, and what a manufacturer's process can consistently achieve - see quality-control testing.

Using Pilot Data to Set Realistic Limits

Specifications are often provisional until pilot batch data is available, since this is the point where real manufacturing variability becomes visible rather than theoretical - see how to scale a chemical product from laboratory to production. It's reasonable to expect specifications to be refined once a manufacturer has run the process and the buyer can see what variation actually occurs at scale.

Working With the Manufacturer to Set Specifications

A good manufacturer will push back on specifications that don't match what its process can reliably achieve, rather than agreeing to limits it can't consistently meet. Treat this as useful technical input rather than resistance - a specification the manufacturer can't reliably hit leads to repeated batch failures, disputes and delay, which serves neither party.

Documenting Specifications Formally

Once agreed, specifications should be documented formally as part of a quality agreement, referenced clearly against test methods and acceptance criteria - see quality agreements. This avoids ambiguity later about what was actually agreed.

Tight Enough, Not Tighter

A specification is the contractual definition of an acceptable product, and the common error is over-specification. Every additional parameter and every narrowed tolerance adds testing cost, increases the chance of a batch being rejected for something no customer would notice, and reduces the number of manufacturers able to meet it.

The useful question for each parameter is what would actually happen if it drifted to the edge of the range. Where the answer is nothing meaningful, the tolerance can widen. Where a customer would notice or performance would suffer, it is doing real work.

Making a Specification Testable

Each parameter needs three things: limits, a test method, and a frequency. Method matters because different techniques give different results for the same property - a viscosity figure without a method and temperature is not testable in any consistent way, and most disputes about whether a batch conformed trace back to exactly that omission.

Agree also what happens when a result falls outside specification: how deviations are investigated, whether retesting is permitted and on what basis, and who authorises release. A process for investigating an out-of-specification result, rather than simply retesting until one passes, is a meaningful indicator of quality culture - see quality control testing.

Questions to Ask a Manufacturer

• What variation does your process typically show for a product like ours?
• Which of our proposed specifications do you think are unrealistic for your equipment?
• Will you help refine specifications after a pilot batch, before they're locked in for commercial production?
• What testing methods do you use, and are they equivalent to any our end market requires?

Frequently Asked Questions

Related Guides

• Quality agreements
• How to scale a chemical product from laboratory to production
• Change-control procedures