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\newcommand{\WPnumber}{37}
\newcommand{\WPtitle}{The Hidden Architecture of Grocery Prices: Promotions, Retail Competition, and the Cost of Living in Quebec}
\newcommand{\WPsubtitle}{Evidence from 300{,}613 posted-price events across 54 Quebec grocery feeds, August--November 2026}
\newcommand{\WPdate}{November 2026}
\newcommand{\WPversion}{1.0}
\newcommand{\WPabstract}{%
Grocery prices are not a single inflation process. Using a read-only research copy of Food-Ka, a Quebec grocery price aggregator, we assemble an auditable panel of 124{,}579 listings from 54 active source feeds (26 retail companies, two channels: online storefronts and weekly circulars) observed every seven hours between 12 August and 1 November 2026 (300{,}613 logged price events, 723{,}140 listing-weeks). We first document a measurement finding relevant to any study of scraped prices: storefronts that serve store-specific prices to anonymous sessions produce intra-day oscillation among three to five price levels, so sub-weekly ``volatility'' in such feeds is a mixture over unidentified stores, not repricing. A calibrated matching procedure (exact UPC within Metro Inc.; brand, package and token-identical names elsewhere; precision 0.91 and recall 0.98 on 4{,}413 UPC-identified pairs, plus a 240-pair reviewed sample) yields 5{,}909 cross-banner groups. On identical products in the same week, the median max--min log price gap across banners is 0.154 (79\% of group-weeks exceed 5\%), banner-pair gaps are persistent (AR(1) 0.66 against a 0.00 permutation placebo), and banner fixed effects range from $-0.20$ (Walmart, Giant Tiger, Maxi) to $+0.12$ (specialty shops) relative to Metro. Inferred promotions cover 4.6\% of storefront listing-weeks with a median depth of 22\%; half of all weekly price changes are promotion entries or exits, doubling measured volatility (ratio 1.92, CI 1.84--2.02). Temporary sales return exactly to their pre-sale reference (mean change $-0.0003$ log points): no reference-price drift within the window; a pre-sale price elevation is mostly a store-mixture artefact and shrinks to about two log points in single-store shops. Lead--lag price associations between rivals are small and fail a lead placebo for cross-company pairs, so no causal response is claimed. Matched-model and naive indices agree within one point at the market level over eleven weeks, but naive averages are off by 10--30 points for banners with assortment turnover. A fixed-basket mixed-integer program shows that unconstrained cheapest-banner shopping saves 15\% relative to the best single banner, 85\% of which two banners capture; store geography and travel costs are unobserved. We publish a five-component Quebec Grocery Price and Affordability Index with uncertainty bands and a prospective collection plan, and report null and failed tests alongside positive ones.%
}
\newcommand{\WPkeywords}{grocery prices; price dispersion; promotions; high--low pricing; web-scraped prices; price indices; record linkage; retail competition; Quebec}
\newcommand{\WPjel}{D22, D40, E31, L11, L81, C81}

% --- Author ---
\newcommand{\WPauthor}{Simon-Pierre Boucher}
\newcommand{\WPaffiliation}{%
  Département des sciences administratives\\
  Université du Québec en Outaouais%
}
\newcommand{\WPemail}{simon-pierre.boucher@uqo.ca}
\newcommand{\WPaddress}{%
  Gatineau -- Pavillon Alexandre-Taché\\
  283, boulevard Alexandre-Taché\\
  Gatineau, Québec, Canada J9A 1L8%
}

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\begin{document}

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\input{sections/introduction}
\input{sections/literature}
\input{sections/institutional}
\input{sections/data}
\input{sections/theory}
\input{sections/methods}
\input{sections/results}
\input{sections/robustness}
\input{sections/policy}
\input{sections/limitations}
\input{sections/conclusion}
\input{sections/reproducibility}

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\input{appendix/appendix}

\end{document}