Short answer. The substantially-all rule is the 80% threshold in the Section 41 four-part test. For a business component to qualify, at least 80% of its research activities, measured by cost or another reasonable basis, has to be elements of a process of experimentation. Meet the 80% and the whole component can qualify. Fall short and the experimentation requirement fails for that component.
What the 80% threshold means
Substantially all is a defined term, and it has a number behind it.
Treasury Regulation section 1.41-4(a)(6) says the process-of-experimentation requirement is met for a business component only if substantially all of its research activities are elements of a process of experimentation for a qualified purpose. Substantially all means 80% or more.
The 80% is measured on a cost basis or another consistently applied reasonable basis, comparing experimentation activities against the component's total research activities. If at least 80% qualifies, the requirement is satisfied for the whole component.
The remaining 20% can be activities that are not themselves experimentation, like some setup, integration, or routine work, without sinking the component.
The shrinking-back rule
If a whole component does not clear 80%, the analysis does not just stop.
When a business component as a whole fails the 80% test, Treasury Regulation section 1.41-4(b)(2) lets you shrink back to a subset of the component, such as a specific element or subsystem, and apply the test there.
Shrinking back means a large product that misses the threshold overall can still produce qualifying research at the level of the part that was genuinely experimental.
Measuring it in practice
The threshold is only as good as the records behind it.
The hard part is not the percentage, it is the measurement. A claim that asserts 80% with no categorized time or activity records invites an examiner to substitute a lower number.
Categorizing the work as it happens, which commits and hours were experimentation versus deployment or administration, turns the 80% from an estimate into something the record supports.
Related references
The 80% rule lives inside the fourth part of the test:
Sources
Every claim on this page traces to a primary authority. Each source below is independent and verifiable.
- Treas. Reg. § 1.41-4(a)(6), substantially all and the process of experimentation - Cornell Law School, Legal Information Institute
- 26 U.S.C. § 41(d)(1)(C), process of experimentation requirement - Cornell Law School, Legal Information Institute
- IRS, Instructions for Form 6765 - Internal Revenue Service
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