Food Inflation Nowcasting with Web-Scraped Prices — Narodowy Bank Polski's Big-Data Model for Real-Time CPI Estimates

Narodowy Bank Polski (NBP), Poland's central bank, has since around 2010 collected high-frequency prices scraped from major online retailers, building an 'eCPI' database that held close to 159 million observed prices for roughly 640,000 products by the end of 2020, growing to around 250 million web-scraped prices by a 2025 peer-reviewed update. NBP's Economic Analysis and Research Department uses this data to nowcast food inflation ahead of the official Consumer Price Index published monthly by Statistics Poland (GUS).

Test whether a web-scraped price index can improve the accuracy of short-term food-inflation forecasts compared with standard time-series benchmarks, and use it to nowcast food inflation before the official CPI release.

NBP Working Paper No. 302 (Macias & Stelmasiak, 2019) backtested the method against random-walk, seasonal random-walk and ARMA/SARMA benchmarks over a pseudo real-time window from January 2014 to June 2018, across 84 elementary COICOP groups and 10 food subaggregates. A 2023 follow-up in the International Journal of Forecasting and a 2025 study in the Journal of Forecasting re-tested the framework through the COVID-19 pandemic and the 2022 food-price shock following Russia's invasion of Ukraine.

Combining the raw web-scraped price index with disaggregated autoregressive distributed-lag (ADL) models cut the root-mean-square forecast error by roughly 27% relative to the best ARMA benchmark, rising to about 30% when forecasts were combined — a reduction found statistically significant using the Diebold-Mariano test. The web-scraped index delivered its largest accuracy gains during the high-volatility COVID-19 and 2022 food-price-shock periods, with only marginal benefit in calmer times.

The published record documents a real, quantified and independently replicated improvement in short-term forecasting accuracy for one narrow but policy-relevant indicator. It does not establish that the eCPI nowcast has itself changed a specific interest-rate decision, and NBP has not published a citizen-facing version of the series.