1# Phase 0 — Feasibility report (WP37)23Date: 2026-11-01 (UTC). Author of record: S.-P. Boucher (UQO), pipeline executed by Claude Code.45## 1. Working directory and repository67- Project created at `Recherche-UQO/economie/hidden-architecture-grocery-prices/` from the UQO working-paper template (`_modele-working-paper`). Nothing pre-existing was overwritten; WP numbers 1–35 are taken, WP36 is an unnumbered stub (`economie/used-car-market-liquidity`), so **WP37** is free.8- Git initialised locally (`main`). Remote `gitsrv:srv/git/wp37_uqo.git` not yet created (creation on spbgit requires the PI's go-ahead; nothing is published externally).9- Mandate stored verbatim in `CLAUDE.md`.1011## 2. Compute, toolchain1213| Item | Observed |14|---|---|15| Machine | Laptop, 18 cores, 48 GiB RAM, 1.0 TiB free (sufficient: raw DB is 165 MB). MacLustr cluster (29 nodes) available but not required. |16| Python | 3.12 venv (`uv`), deps pinned in `requirements-lock.txt` (pandas, statsmodels, linearmodels, pyfixest, scikit-learn, lightgbm, duckdb, pulp, matplotlib, pytest…) |17| R | 4.x available (`/opt/homebrew/bin/R`) — fallback only |18| LaTeX | TeX Live, latexmk 4.88 ; pandoc present |19| Seeds | `config/settings.yaml: seed: 20261101` used by every stochastic step (bootstrap, CV folds, sampling) |2021## 3. Data access — what actually exists2223**Food-Ka is owned by the PI**; the production SQLite database (`M2U64:~/apps/food-ka/data/foodka.db`, WAL mode) was copied with a consistent `.backup` from a read-only connection (production untouched). Checksum verified. Provenance in `data/raw/README.md`.2425Public cross-check (2026-11-01 ~08:00 UTC): homepage shows 50 760 products, 54 banners, 8 631 on promotion — identical to the snapshot's `active=1` rows. The mandate's figures (50 886 / 54 / 8 590) were the homepage values a few hours earlier; they are **source-feed listings, not unique products**. `robots.txt` disallows `/api/` to crawlers; the API was queried once for this cross-check only and is not a data source.2627### 3.1 Schema (observed)2829`products` (124 579 rows: 50 760 active, 73 819 deactivated) — uid = `source:external_id`, name, brand, category (18 canonical), category_raw, size_label, price, regular_price, price_label, on_sale, unit_price, unit_price_label, in_stock, details (JSON), first_seen, last_seen, updated_at, miss_count, active.30`price_log` (300 613 rows: uid, ts, price) — **event log**: one row at first observation and one row at every *change* of `price` (NULL = price disappeared from display). No row when the price is unchanged. `regular_price`, `on_sale`, `in_stock` are **not** historised.31`sync_log` (15 630 rows) — one row per connector pass (57 sources × ~255–308 passes, median gap **7.2 h**, 2026-08-12 → 2026-11-01), with found/added/updated/removed counts.32`product_links` (2 537 uids in 1 103 cross-banner groups) — Food-Ka's own brand+size+name matching (not GTIN based).33`off_cache` (25 630) — Open Food Facts barcodes attached by a conservative brand/name/size lookup (derived, not source-provided).3435### 3.2 Data-generating process (from the connector code, read-only copy in `data/raw/foodka-src/`)3637- 57 active source feeds = **54 with active products**, belonging to far fewer **retail companies** (Loblaw, Metro Inc., Sobeys/Empire, Walmart, Costco, independents, four pharmacy banners, the SAQ monopoly); several banners appear twice (online storefront feed + Flipp circular feed). A `retailers` table (company → banner → feed → channel) is built in Phase 1 and ownership is verified against public corporate information in Phase 2.38- **Two channels**: (a) *online storefront* feeds (Shopify/WooCommerce JSON, Loblaw/Metro/Walmart/Costco/IGA sites scraped through residential proxy pools without store selection); (b) *weekly circulars* via Flipp for postal code **H2X 1Y4 (Montréal)** with `valid_from`/`valid_to`, `original_price`, `dollars_off`, fine print.39- **Geography**: no store identifier anywhere. Circulars are regional (Montréal zone); online prices are whatever store the retailer's site assigned to the proxy session. → **Store-level geography is unavailable** (H4 and travel-cost components of Model E cannot be estimated from Food-Ka).40- **Identifiers**: Metro and Super C `external_id` are valid GS1 UPC-A codes (100 % check-digit valid, n = 1 954 active). Shopify/Walmart/Loblaw ids are internal SKUs (check-digit validity ≈ 10 %, i.e. chance). Exact GTIN matching is therefore only possible within Metro Inc.; everything else requires validated probabilistic matching.4142### 3.3 Critical measurement finding — store-mixture noise in scraped banners4344For anti-bot banners (club_entrepot, metro, maxi, superc, provigo, walmart) the event log shows **high-frequency oscillation among a small set of price values** (e.g. Metro mayonnaise 890 mL alternating 5.77 / 6.99 / 7.49 / 7.79 / 7.99 within the same day; 54–68 % of change events revert to the previous-but-one value; median 4 distinct values per product; week-to-week Jaccard of value sets ≈ 0.5). This is consistent with **each scrape being served by a different (unknown) store**, not with genuine intra-day repricing. Consequences, to be handled in Phase 1 and preregistered in Phase 3:45- a banner-level "posted price" at time *t* must be defined as a statistic of the daily/weekly value set (mode, max = likely shelf regular, min = likely best store/promo), with sensitivity analysis;46- intra-week volatility for these banners is **not** interpretable as price flexibility; H2 must be tested on weekly aggregates and separately on circular feeds (clean weekly cycles);47- the amplitude of the value set is itself a (store-unidentified) lower bound on **within-banner cross-store dispersion** — a usable, honestly-labelled quantity.48Shopify/WooCommerce feeds (single online store) show far fewer events and no such cycling.4950### 3.4 Variable availability (summary; full inventory in Phase 1)5152| Variable | Status |53|---|---|54| product name, brand, category, size_label | confirmed (brand missing for several feeds: walmart 87 %, adonis/tt/akhavan/bocoboco 100 %) |55| UPC/GTIN | confirmed for Metro/Super C only; derivable-unreliable via OFF barcode elsewhere |56| unit price ($/100 g, $/100 ml, $/un) | derivable (Food-Ka's parser); missing where size_label absent (giant_tiger 92 %, nuvo 100 %, most Flipp feeds) |57| current price history | confirmed (event log, 81 days) |58| regular/reference price history | **unavailable** (current state only); circulars carry `original_price` per item-week |59| promotion flag history | unavailable → must be **inferred** from price paths; validated against current `on_sale` and circular items |60| observation timestamps | confirmed (`sync_log` per source; product-level presence between events inferred) |61| circular validity | confirmed (Flipp feeds) |62| availability/in_stock | current only; unreliable (NULL for 23 %) |63| store / geography | **unavailable** |64| quantities / expenditure weights | unavailable (no transactions) |6566## 4. External benchmarks6768Statistics Canada WDS reachable: table 18-10-0004-01 (CPI monthly NSA, Quebec = member 11, Montréal = 13; "Food purchased from stores" = member 4; latest 2026-09) and 18-10-0245-01 (monthly average retail prices for 110 selected products, Quebec; latest 2026-08). Overlap with the Food-Ka window is only **Aug–Sep 2026** (2 months) — validation of QGPAI against official indices will necessarily be weak and labelled as such.6970## 5. API credentials (minimal tests, 2026-11-01T08:02Z)7172| Provider | Model | Result |73|---|---|---|74| Anthropic | claude-opus-5-5, claude-sonnet-5-5, claude-haiku-5-5 | OK (end_turn, 18 in / 4 out tokens each) |75| OpenAI | gpt-5.4, gpt-5.4-mini, gpt-5.4-nano | OK |7677Planned LLM use is limited and auxiliary: (i) second annotator for the stratified match-validation sample (human = PI adjudication remains the reference), (ii) nothing in estimation. Every call is logged with model id, tokens and cost.7879## 6. GO / NO-GO gates8081| Gate | Decision | Basis |82|---|---|---|83| G1 Authorised data access | **GO** | owner export, read-only copy, checksum |84| G2 Longitudinal panel | **GO (partial)** | 81-day event log, ~3.3 passes/day/source; reconstructable daily panel; regular-price history absent |85| G3 Cross-banner matching | **GO (partial)** | exact UPC only Metro↔Super C; probabilistic matching + validation elsewhere; expect a few thousand matched products, not 50 k |86| G4 Promotion dynamics | **GO (partial)** | circulars: clean weekly cycles with reference prices; online feeds: promotion state inferred from price paths and store-mixture noise |87| G5 Competitive spillovers | **GO descriptive only** | weekly banner-level responses; no exogenous variation identified a priori → no causal claims |88| G6 Store geography / travel costs | **NO-GO** | no store identifiers; H4 untestable; Model E limited to banner-count constraints; prospective plan required |89| G7 Price indices vs StatCan | **GO (weak)** | 2-month overlap only |90| G8 Compute | **GO** | laptop sufficient |9192## 7. Expected compute9394Whole pipeline (panel reconstruction, matching, fixed-effects models, bootstraps, MIP baskets, rolling-origin forecasts) is expected to run in well under one hour on the laptop; no cluster job required.95