Additionality

Attribute only the outcome increment that would not have occurred without an intervention, using an explicit counterfactual baseline and adjustments for displaced, leaked, substituted, or already-planned activity.

Version

v2 · 2026-09-06 · History

Domain-specific #

1241

Origin domain

economics

Subdomain

program evaluation

Aliases

Additional impact, Incremental impact, Project additionality

Core Idea

Additionality is the property that an intervention causes an outcome beyond what would otherwise have occurred. If \(Y_1\) denotes the relevant outcome with the intervention and \(Y_0\) the counterfactual outcome without it, the simplest expression is \(A=Y_1-Y_0\). Only \(Y_1\) can ordinarily be observed for treated units at the relevant time; \(Y_0\) must be estimated or justified. The credibility of additionality therefore rests on the baseline and causal design, not on the arithmetic or the fact that activity occurred after support was announced.

Scope of Application

Additionality is literal when an evaluator attributes a defined outcome increment to an intervention relative to a credible without-intervention baseline and accounts for relevant offsetting effects.

• Public-program appraisal. Forecast net jobs, housing, services, or social value before funding.
• Impact evaluation. Estimate realized causal effects after implementation.
• Carbon crediting. Screen whether mitigation depends on the crediting intervention and quantify a defensible increment.
• Conservation finance. Distinguish protected outcomes caused by support from outcomes already expected.
• Development finance. Assess whether public or concessional capital changes scale, timing, risk, or beneficiaries.
• Research funding. Ask which knowledge or capacity would be absent without an award, with caution about speculative baselines.
• Impact investing. Separate enterprise impact from investor contribution.
• Place-based policy. Adjust local gains for displacement, leakage, and wider-area effects.

Clarity

Name the intervention, decision unit, outcome, measurement period, geographic and beneficiary boundary, baseline, causal assumptions, and assessment mode. State whether additionality concerns existence, scale, quality, timing, or location. Separate project additionality from the additionality of one funding source and from the quantity credited. Explain how policy requirements, common practice, profitability, prior commitments, and other finance enter the baseline. Quantify deadweight, displacement, leakage, substitution, multipliers, and interactions only where supported, and report sensitivity rather than hiding uncertainty in one number.

Manages Complexity

Programs generate visible outputs amid trends, private incentives, other policies, spillovers, and strategic behavior. Additionality manages this complexity by forcing a two-world comparison and decomposing the gap between gross and net effects. The baseline creates a common attribution frame; deadweight, displacement, and leakage locate predictable overstatement. Yet the unobserved world can be manipulated, especially when credits or funding depend on passing a test. Standardized tests improve consistency but trade project specificity for administrability.

Abstract Reasoning

1. Define the intervention and the exact outcome whose increment is claimed. 2. Fix the population, place, time horizon, and unit of attribution. 3. Construct the most credible without-intervention baseline using declared evidence. 4. Estimate the treated or intervention scenario on the same outcome scale. 5. Subtract baseline outcome from intervention outcome and preserve uncertainty. 6. Remove deadweight and account for displacement, leakage, and substitution where relevant.

Knowledge Transfer

Counterfactual Subtraction is the strict parent by specialization. Additionality estimates an intervention's effect by subtracting a constructed no-intervention baseline from the treated scenario. The parent contributes potential worlds, baseline credibility, subtraction, and causal uncertainty. The policy residual is eligibility, deadweight, displacement, leakage, common-practice and legal-requirement tests, and crediting governance.

Relationships to Other Abstractions

Current abstraction Additionality Domain-specific

Parents (1) — more general patterns this builds on

• Additionality is a kind of Counterfactual Subtraction Prime

  Counterfactual Subtraction is the strict parent by specialization: every additionality claim depends on an explicit no-intervention baseline and the increment above it.

Hierarchy paths (2) — routes to 2 parentless roots

• Additionality → Counterfactual Subtraction → Counterfactuals → Causality → Dependency

Show alternative path (1)

Neighborhood in Abstraction Space

Additionality sits in a sparse region of the domain-specific corpus (93^rd percentile for distinctiveness): few abstractions share its structure, so a faithful description tends to retrieve it precisely.

Family — Unclustered & Miscellaneous (1565 abstractions)

Nearest neighbors

• Causal Inference — 0.79
• Population Health — 0.79
• Propensity score matching — 0.78
• Differential effects — 0.78
• Selection on Observables — 0.77

Computed from structural-signature embeddings · 2026-09-08