How the data is made

The Vynn Canadian Grocery Price Index is a chained, matched-model Jevons index computed over reference stores, joined at event time to the designation in force when each price was observed — a later panel change (a store failover, a re-designation) never rewrites published history.

• Each link joins a week to the most recent preceding week that had usable observations — normally the week before, but longer across a collection gap. Only items priced in both of those weeks contribute. This is the “matched model”: it prevents a change in which products happened to be collected from registering as a price change.
• Links are therefore between consecutive measured weeks, not necessarily consecutive ISO weeks. weeks_spanned records the real calendar separation, so a multi-week link is never mistaken for a one-week move.
• The link between two weeks is the geometric mean of price relatives across matched items. Geometric rather than arithmetic, so a price and its reversal cancel exactly.
• Links are chained onto a running index. The base week is 100.
• An item is keyed by banner, store and SKU, so the same product at two retailers — or at two stores of one banner — is two items. Neither a price difference between retailers nor a change in geographic sample composition is ever read as a price change.
• Only closed ISO weeks aggregate. The in-progress week is mid-collection by construction, so it publishes no measurement and reports carried-forward instead of a value that would drift as batches land.
• The base seats at the first week with at least the minimum matched items: a thin first week cannot become the permanent base, and with no adequate week the series is reported unavailable rather than a flat 100.
• A week with fewer than 30 matched items is suppressed, not published: the previous value is carried forward and the point is flagged carried_forward.
• A week with no usable observations publishes no measurement. It carries the previous value, is flagged, and does not become the base of the next link. The series always has one point per calendar week, so week-over-week and year-over-year offsets mean what they say.
• When a link spans a gap, weeks_spanned reports the real interval and the week-over-week figure is withheld — a multi-week move is not a one-week move.

What this is not. It is not an official consumer price index. It is not weighted by expenditure share, and it is not adjusted for quality change. It is a transparent, reproducible measure of observed shelf prices across a defined panel.