The fund's primary metric, computed to GiveWell standard, with every assumption, adjustment, and cost basis visible.
Most cost-effectiveness claims in this sector are a single number with an invisible denominator. This annex does the opposite: it publishes the machinery. All seven programs in the portfolio are modeled end-to-end, each on an honestly stated cost basis: the flagship worksheet is below, and every program has its own worksheet page and downloadable workbook in the portfolio section.
Every program in the portfolio is scored on the same four metrics. Together they answer the two questions a funder should insist on: what does a dollar buy, and would the claim survive a referee.
Each model also reports self-sustainability: the ratio of carbon revenue to delivery cost, in the base case and at the contracted downside price floor. Moral-weight sensitivity is run under three published frameworks (GiveWell default, egalitarian, wellbeing-anchored); the GiveWell default is the headline.
The flagship school safe-water program: 2,000 schools and 1.98 million students at scale, Gold Standard registered, Article 6 credits contracted and issuing since 2024. The model below is deliberately strict with itself.
All-in delivery cost over 2024–2033: filtration hardware and replacements, rainwater systems, and in-country staffing, $12.25M against 14.7M student-years served ($0.83 per student-year). Health benefit is built bottom-up: diarrheal illness averted among students only, valued as years lived with disability, using the meta-analytic filtration effect (Wolf et al. 2022, Lancet) and sensor-verified delivery performance.
Under-five mortality and household spillovers (the effects measured in Rwanda's cluster-randomized trial, Kirby et al. 2019), respiratory-infection reductions, and fuel and time savings (the dominant benefits in Barstow et al. 2019). Every exclusion is conservative: each would lower the cost per DALY, some substantially. They enter as the evidence is traced, not before.
Why publish a $6,822 figure when the site's headline is under $100? Because they answer different questions on different bases, and this annex exists to keep those bases separate. The $6,822 is the strictest possible gross frame: school-age illness only, all costs in, every unproven benefit out. The stronger gross figures live in the portfolio's community programs, where under-five exposure dominates and the randomized mortality-relevant evidence applies: Asili DR Congo models at $355 per DALY on its philanthropic cost basis, and Burundi reaches $2,908 under the Kremer mortality scenario on full cost; every program's worksheet is linked in the portfolio section. And the fund's primary metric, marginal donor cost, is what carbon revenue drives toward zero regardless of frame.
Everything below is the complete v1 workbook: every input with its provenance, every calculation in the order it runs, every scenario. Nothing is summarized away.
| Series | 2024 | 2025 | 2026 | 2027 | 2028 | 2029 | 2030 | 2031 | 2032 | 2033 | Total |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Students enrolled | 197,600 | 444,600 | 839,800 | 1,383,200 | 1,976,000 | 1,976,000 | 1,976,000 | 1,976,000 | 1,976,000 | 1,976,000 | 14,721,200 student-yrs |
| Net credits (tCO₂e) | 7,282 | 32,215 | 37,440 | 69,887 | 99,838 | 99,838 | 99,838 | 99,838 | 99,838 | 99,838 | 745,852 |
| Carbon revenue ($) | 0 | 109,230 | 515,440 | 636,480 | 1,257,966 | 1,896,922 | 1,996,760 | 2,096,598 | 2,196,436 | 2,296,274 | 13,002,106 |
| Rainwater capex ($) | 250,000 | 300,000 | 450,000 | 550,000 | 250,000 | 0 | 0 | 0 | 0 | 0 | 1,800,000 |
| Filter capex ($) | 852,891 | 1,245,221 | 234,423 | 994,422 | 1,084,824 | 0 | 0 | 0 | 0 | 0 | 4,411,781 |
| Replacement filters ($) | 24,700 | 55,575 | 104,975 | 172,900 | 247,000 | 247,000 | 247,000 | 247,000 | 247,000 | 247,000 | 1,840,150 |
| In-country staffing ($) | 420,000 | 420,000 | 420,000 | 420,000 | 420,000 | 420,000 | 420,000 | 420,000 | 420,000 | 420,000 | 4,200,000 |
All rows are proforma inputs from the base scenario of the portfolio financial model; net credits carry the ~5% Article 6 host-country retirement already deducted, on the Gold Standard SDWS V2.0 yield basis (0.053 credits per person-year). Credits issued to date: 16,153 tCO₂e (Sep 2024) plus 17,758 pending, matching the 2024–25 rows.
| TOTAL_PROGRAMME_COST, 2024–2033 | $12,251,931 | Formula rainwater + filter capex + replacements + staffing |
| Student-years served | 14,721,200 | Formula sum of the enrollment row |
| Cost per student-year | $0.832 | Formula cost ÷ student-years |
Every figure on this page divides by this one cost. The build fails programmatically if any downstream number uses a different denominator. Deliberately excluded from this cost: an allocated share of portfolio-level management, verification, and sensor operations (an optimistic omission, priced in scenario S7), and post-2033 operating years within the 15-year crediting period (a conservative omission: the steady-state years are the cheapest). The organization's published "∼$12M investment" reconciles with this basis as a cross-check; it is not the denominator.
| Input | Value | Provenance | Basis |
|---|---|---|---|
| Diarrheal incidence, school-age | 0.60 episodes/child-yr | Judgment · flagged | Country-specific GBD lookup pending; tested 0.30–1.20 in scenarios S1–S2. The widest single uncertainty in the model. |
| Diarrhea reduction, point-of-use filtration | 34% | Evidence-anchored | Wolf et al. 2022 (Lancet), filtration risk ratio 0.66. The program's own randomized result (29%, Kirby et al. 2019) is consistent; the meta-analytic value is used, and a conservative 25% is tested in S5. |
| School share of daily drinking water | 30% | Judgment · flagged | Students drink roughly a third of their water at school; no time-use survey yet. Tested 20–40% in S3–S4. |
| Delivery effectiveness (safe at the point) | 90% | Sensor-measured | Portfolio monitoring record: 90.3% of samples safe under monitored delivery (n=1,685) against 24.3% at baseline (n=4,943). |
| Disability weight, diarrheal episode | 0.188 | Evidence-anchored flagged | GBD moderate diarrhea. The mild/moderate severity mix is unresolved (mild is 0.074), so this choice is flagged. |
| Episode duration | 4.3 days | Judgment | Typical acute episode. |
| YLD per episode | 0.002215 DALY | Formula | 0.188 × 4.3 ÷ 365 |
| Step | Value | Computation |
|---|---|---|
| Episodes averted, 2024–2033 | 810,844 | 14,721,200 student-yrs × 0.60 × 0.34 × 0.30 × 0.90 |
| DALYs averted (raw) | 1,796 | episodes × 0.002215 YLD. Years lived with disability only: school-age diarrheal mortality is excluded (conservative). |
| $ / DALY averted, raw | $6,822 | $12,251,931 ÷ 1,796 |
| Evidence adjustment (section E) | × 0.486 | 1,796 → 874 DALYs adjusted |
| $ / DALY averted, evidence-adjusted | $14,026 | $12,251,931 ÷ 874 |
| xCash, GiveWell default weights | 0.101× | DALYs × 2.3 units ÷ cost ÷ 0.00335 units/$ (GiveDirectly benchmark) |
| xCash, egalitarian / wellbeing weights | 0.044× / 0.158× | same chain, DALY moral weight 1.0 / 3.6 |
| xCash, moral-uncertainty weighted | 0.103× | 0.45 × GiveWell + 0.25 × egalitarian + 0.30 × wellbeing. Ranking is stable across all three frameworks. |
| Factor | Score | Basis |
|---|---|---|
| Internal validity | 0.80 | The effect anchor is meta-analytic, but the chain multiplies it by two judgment inputs (incidence, school share). |
| External validity | 0.80 | Household point-of-use trials transferred to school gravity-filter delivery. |
| Publication bias | 0.80 | Systematic review without an independent funnel-plot confirmation in this model. |
| Implementation quality | 0.95 | Sensor-instrumented delivery, continuous monitoring record, registry issuance track record. |
| Composite | 0.486 | Formula product of the four. Reported alongside the raw figure, never silently multiplied into it. |
| Residual baseline-practice factor | 0.78 | Judgment · flagged The 2026 methodology reconstruction (V2.0) already cut the crediting basis by ~45%; this prices the residual risk that baseline water-boiling practice is still overstated. A measured school baseline survey replaces it. |
| Delivery effectiveness | 0.90 | Sensor-measured The same cell the health chain uses. One usage number, two consumers. |
| Leakage / rebound | 1.00 | Negligible for water treatment. |
| Credit integrity ratio | 0.70 | Formula 0.78 × 0.90 × 1.00 |
| $ / tonne, credited basis | $16.43 | $12,251,931 ÷ 745,852 t · 11.3× below the $185 social cost of carbon |
| $ / tonne, integrity-adjusted | $23.47 | $12,251,931 ÷ (745,852 × 0.70) · 7.9× below the SCC. Fails the SCC test only if integrity falls below 0.089. |
| Implied realized price | $17.43/t | $13,002,106 revenue ÷ 745,852 t, blended across the Article 6 offtake and post-contract sales |
| Revenue coverage of full cost | 106.1% | $13,002,106 ÷ $12,251,931 → net philanthropic cost −$750,175: self-financing over the window |
| Steady-state coverage | 2.84× → 3.44× | 2029 and 2033 revenue against the $667,000 steady-state year (replacements + staffing) |
| Scenario | Varied input | DALYs | $ / DALY raw | xCash GW | xCash MU | Reading |
|---|---|---|---|---|---|---|
| Base case | — | 1,796 | $6,822 | 0.101× | 0.103× | the strict frame, as above |
| S1 · incidence low | 0.30 | 898 | $13,645 | 0.050× | 0.052× | worst single-input case; climate finding unchanged |
| S2 · incidence high | 1.20 | 3,592 | $3,411 | 0.201× | 0.207× | — |
| S3 · school water share low | 20% | 1,197 | $10,234 | 0.067× | 0.069× | — |
| S4 · school water share high | 40% | 2,394 | $5,117 | 0.134× | 0.138× | — |
| S5 · conservative effect size | 25% | 1,320 | $9,278 | 0.074× | 0.076× | GiveWell-style conservative reduction |
| S6 · + fuelwood savings | $5.2M / 10 yr | 1,796 | $1,308 -equivalent | 0.525× | 0.528× | 13,000 t/yr at $40/t would dominate health value 4:1. Unverified incidence; scenario-only until measured, never central. |
| S7 · + allocated portfolio overhead | +$2.5M | 1,796 | $8,214 | 0.084× | 0.086× | cost per tonne rises to $19.78, still 9.4× below the SCC |
| S8 · downside price floor | $10/t | 1,796 | $2,669 net basis | coverage 60.9% | revenue $7.46M, net philanthropic cost $4.79M; the donor-cost ladder's third rung | |
Recomputed robustness statement, not asserted: no single input moves the health frame anywhere near the cash-transfer bar on full cost, and none reverses the climate finding. The self-financing conclusion is the fragile one, and its reversal condition is stated in section F and S8.
| Dimension | Rating | Assessment |
|---|---|---|
| Effect-size quality | The meta-analytic anchor is solid, but the chain multiplies it by two judgment inputs with no Rwanda-specific data, and the program's own randomized result is not yet independently traced into the model file. Preliminary status is correct; the burden lookup and source tracing gate the upgrade. | |
| Counterfactual robustness | Health-side funging is low: no other funder provides school point-of-use treatment at this scale in Rwanda. Carbon-side additionality is the sharper question: a registered project with a contracted buyer could plausibly attract another developer, and v2 must model it. | |
| Moral-weight transparency | All frameworks reported; ranking stable. The decision-relevant fact is the frame (gross vs marginal donor cost), not the framework. | |
| Attribution and integrity | Virridy develops and operates the program, so attribution near 1.0 is defensible. The integrity ratio is decomposed and shares its delivery term with the health chain; its weak link, the 0.78 baseline residual, is flagged, not hidden. | |
| Excluded factors | Excluded: under-five and household spillovers, respiratory effects, fuel and time savings (conservative on benefits); allocated overhead and post-2033 years (boundary choices priced in S7 and noted in B). Net direction: conservative on benefits, optimistic on the cost boundary. | |
| Ranking stability | Health and climate frames are stable across S1–S8. The self-financing finding is the sensitive one: it fails below roughly $16.40/t average realized price or a 6% issuance shortfall. |
The same program, the same DALYs, three honest cost bases. The metric that matters to a donor is the third.
Even the downside row is conservative twice over: it holds the strict students-only health frame, and it prices every credit at the contracted floor. The reversal condition is explicit in the model: self-financing fails if realized prices average below roughly $16.40 per tonne or issuance falls more than 6% below the base case.
The protocol adopts the standard machinery of professional cost-effectiveness analysis, then adds discipline specific to carbon-financed delivery.
Bottom-up disease-burden construction (never a percentage applied to an aggregate burden). A four-factor evidence-adjustment pipeline with published scoring rubrics, reported alongside the raw estimate, never silently multiplied in. Three moral-weight frameworks run in parallel. Counterfactual and funging analysis with named alternative funders. One-way sensitivity on every load-bearing input, with probabilistic analysis reserved for models whose inputs can honestly carry distributions.
A single cost denominator per model, asserted in code so the build fails if any figure divides by a different cost than the one that produced the benefit. One delivery number shared by the health chain and the credit quantification. An integrity ratio that decomposes how credited tonnes relate to real ones, informed by the continuous sensor record rather than annual surveys. And a hard labeling rule: revenue-leveraged figures are never presented as gross cost-effectiveness.
Every model faces a three-seat referee panel before its numbers are cited: an econometrician (recomputes every chain from raw inputs), an uncertainty analyst (attacks the sensitivity and scenario structure), and a skeptical grantmaker briefed to argue against funding, who for this portfolio also takes the carbon-market critic's chair: suppressed-demand baselines, self-reported usage, and additionality are attacked before an outside critic does. The worked example above is v1 and enters that review now; the independent, GiveWell-partnered evaluation in design supersedes all of it.
Delivery inputs draw on the portfolio's continuous verification layer: 1,273 monitored water sites, with pooled water quality moving from 24% of samples testing safe at baseline (n=4,943) to 90% under monitored delivery (n=1,685). See the evidence base on the main page. Sensor credibility is applied only where the sensor actually measures: water safety at the point of delivery, not household behavior, and the models keep that boundary explicit.
Every program is modeled with the same protocol; each row links to that program's full cell-by-cell worksheet and downloadable workbook. The cost bases differ by design, so the rows are labeled rather than averaged: a single portfolio $/DALY would mix denominators, and this annex does not do that.
| Program | Cost basis (2024/25–2033) | $ / DALY raw → net donor | $ / tCO₂e (integrity-adj) | Integrity | Worksheet |
|---|---|---|---|---|---|
| Amazi Meza · Rwanda | Full delivery · $12.25M | $6,822 → $0 (coverage 106%) | $16.43 ($23.47) · 7.9× under SCC | 0.70 | on this page |
| Amazi Water · Burundi | Full delivery · $53.4M | $5,307 → $2,450 ($15 floor) | $27.87 ($44.23) · 4.2× | 0.63 | worksheet |
| Asili · DR Congo | Philanthropic rehab · $1.2M | $355 ($1,133 adj); tariffs fund O&M | $1.08 ($2.30) · 80× | 0.47 flagged | worksheet |
| LifeStraw · Kenya | Carbon financing only · $2.35M | $7,312 financing basis, labeled | $18.21 ($25.29) · 7.3× | 0.72 | worksheet |
| MWA DRIP · Kenya | Grant + purchases · $7.49M | none by design pathway not yet designed | $13.79 ($18.03) · 10.3× | 0.77 | worksheet |
| Helvetas · Madagascar | Carbon financing only · $2.75M | $572 financing basis, labeled | $10.31 ($20.21) · 9.2× | 0.51 flagged | worksheet |
| Water Mission · Tanzania | Offtake outlay · $10.39M | none by design another org's delivery | $12.83 ($17.82) · 10.4× | 0.72 | worksheet |
What rolls up honestly across bases: 5.5M tonnes modeled across the seven programs, 2024–2033, and every program's integrity-adjusted cost per tonne sits between 4.2× and 80× below the $185 social cost of carbon on its own stated basis. What does not roll up: a single portfolio $/DALY (mixed denominators), and the strongest per-DALY rows (Asili, Helvetas) are the ones on partial cost bases, which is exactly why the basis column exists. Integrity flags: the three programs still on pre-reconstruction crediting baselines (Asili 0.243, Helvetas 0.21, Burundi 0.170 tonnes per person-year) carry the deepest integrity haircuts here, quantified before any critic asks. All models are v1 preliminary, in adversarial review.