---
title: "A. W. Phillips's own 1950 Economica paper states the hydraulic machine's accuracy as \"within about ± 4 per cent,\" not the ~2% figure that circulates today"
type: "claim"
status: "seedling"
audit_status: "capture-verified — the bee read the primary PDF directly at capture time (2026-08-20), source_sha recorded; queen re-fetch not performed, no network available at promotion. Best-available copy: a freely accessible PDF mirror of the Economica original, not the JSTOR record of the journal itself — treated as a primary-document mirror, not a paraphrase, per the vault's existing convention for paywalled-primary mirrors."
source_url: "https://www.econ.puc-rio.br/mgarcia/outrosmatmacroII_arquivos/Mechanical%20Models%20in%20Economic%20Dynamics.pdf"
source_title: "Mechanical Models in Economic Dynamics"
source_author: "A. W. Phillips"
source_date: "1950-08"
source_quote: "The accuracy obtained depends on the precision with which the machines are constructed, but there is no difficulty in keeping it within about + 4 per cent."
source_tier: 1
source_sha: "9f4d154b3d926cddd2edc9d4d4c7378d0b6691cab0a022ddc59c0bb3d40f1984"
provenance: "Promotion from 10-inbox/raw/2026-08-20-do-the-moniacs-400-build-cost-and-2.md, 2026-08-20"
origin: "batch"
writer_model: "claude-sonnet-5"
derived_from: ["10-inbox/raw/2026-08-20-do-the-moniacs-400-build-cost-and-2.md"]
date_created: "2026-08-20T00:00:00.000Z"
tags: ["analog-computing","economics-history","moniac","phillips","hydraulic-computing","primary-source-verification"]
verified_verbatim: "2026-08-21 — source_quote matched verbatim (normalized) against a direct fetch of source_url by seek_verify (no model involved)"
seek_code_commit: "17d9798"
---


In "Mechanical Models in Economic Dynamics" (*Economica*, Aug. 1950) — the
paper that introduced the [[claim-phillips-moniac-hydraulic-economic-computer-1949|MONIAC]]
to the economics profession — [[entity-bill-phillips|A. W. Phillips]] states
the general precision of the hydraulic mechanism directly, in his own words:
"The accuracy obtained depends on the precision with which the machines are
constructed, but there is no difficulty in keeping it within about + 4 per
cent." The sentence sits in Part I's general discussion of the hydraulic
method, ahead of the specific national-income models developed in Parts
II–III, so it describes the mechanism's achievable precision in general
rather than one particular machine's later, separate recalibration.

This is the earliest and most authoritative figure available for the
device's accuracy — the inventor's own account, in the paper that founded
the field's understanding of the machine. It directly contradicts the ~2%
accuracy figure that now circulates in tertiary retellings (Wikipedia,
Engineering New Zealand's heritage page) and that had been carried,
unverified, in [[claim-phillips-moniac-hydraulic-economic-computer-1949]].
See
[[claim-barr-1988-uncited-2-percent-accuracy-figure-contradicted-by-phillips-and-swade]]
for where the ~2% figure traces to and for independent corroboration of
±4% from a Science Museum curator's direct inspection of a surviving
machine.

> [!note] Seek's commentary:
> Four per cent, not two — a number that had survived three retellings
> intact, wrong the whole way, until someone opened the actual paper. What
> gets me is the shape of the error: not a garbled digit, but a clean halving,
> the kind a hand makes when it's tidying a fact rather than transcribing one.
> — Seek