Kepie Watch / The Grocery Index / Method
Method · The Grocery Index
How we measure a household’s grocery week.
One basket. Two supermarkets. Five household sizes. Updated every Sunday.
1. What the Index measures (and what it doesn’t)
The Kepie Grocery Index is a single weekly number: what it costs a New Zealand household of a given size to fill its grocery week at one supermarket. The figure covers in-home grocery food only — the same scope Stats NZ uses for the “grocery food”, “fruit and vegetables”, “meat, poultry and fish” and “non-alcoholic beverages” categories of the Food Price Index. That accounts for roughly three-quarters of what a household spends on food each week.
The Index does notinclude restaurants, cafes, takeaways, ready-to-eat meals, alcohol, or delivery services. Those are real costs, but they sit outside what a weekly supermarket shop captures — and the Index is built to be a clean supermarket-shop number that families can compare against their own receipts.
2. The basket
The basket is the 9 New Zealand staples Kepie Watch tracks every week. Each item carries a standard weekly quantity for a one-person household; the household-size table below scales the whole basket up from there.
| Staple | Weekly quantity (per person) |
|---|---|
| Butter | 500g |
| Cheddar cheese | 500g |
| Eggs | 1 dozen |
| Bread | 1 loaf (~700g, rounded to kg) |
| Chicken thigh | 1kg |
| Beef mince | 1kg |
| Bananas | 1kg |
| Capsicums | 500g (~3 capsicums) |
| Broccoli | 400g (~1 head) |
As Kepie Watch adds more staples to its weekly tracking, the basket grows. The methodology version on file with each Sunday’s publication records exactly what was in that week’s basket — nothing back-fills, nothing rewrites.
3. The picker
For each basket item at each supermarket, the Index takes the cheapest pack we’ve confirmed is actually that staple. Branded, house-brand, organic, free-range, different pack sizes — everything that genuinely is the staple goes into the comparison. A tub of buttery spread does not count as butter; a bag of bread mix does not count as a loaf. The same exclusion list that powers the staple pages drives the Index — see the Kepie Watch method for the longer description of how we keep that list honest.
4. The basis
Every comparison runs on a unit basis — per kilo, per litre, or per each — never on pack price. A 1kg block of butter and a 500g block sit on the same scale: cost per kilo. A 2L milk and a 1L milk: cost per litre. A dozen eggs and a half-dozen: cost per egg. That keeps small packs from looking artificially cheap and big packs from looking artificially expensive.
We arrived at this rule the hard way — an earlier draft of one staple page crowned the wrong winner because it compared pack prices instead of per-kilo. The published rule, baked into a continuous-integration test, is that no Kepie Watch surface (the Index included) may show a headline winner derived from pack price.
“We measure the shelves. We don’t opine on what we find.”
Kepie Watch · The Grocery Index
5. Household-size scaling
Five household sizes share one basket. The per-person basket is multiplied by the scaling factors below to produce the cost for two, three, four, and five-person households. The factors reflect what shared kitchens actually shop — the per-person cost falls as you add people because staples like a 700g loaf of bread or a 1kg block of cheese are shared, not bought five times over.
| Household size | Scaling factor | Why |
|---|---|---|
| Household of 1 | 1.0× | Single-person baseline. |
| Household of 2 | 1.7× | Shared staples; couples buy one loaf, not two. |
| Household of 3 | 2.4× | One child added; cheaper-per-head than two. |
| Household of 4 | 3.1× | Standard NZ family of four; the audience anchor. |
| Household of 5 | 3.6× | Diminishing per-person cost as the kitchen scales. |
The factors come from the same household-spending research that drives Kepie’s onboarding budget buckets — New Zealand-focused work from MoneyBalance and Steady NZ, cross-checked against Stats NZ’s Household Economic Survey. They’re reviewed when new data lands.
6. The Food Price Index overlay
The Index uses the same in-home grocery scope as Stats NZ’s Food Price Index. Restaurants, takeaways and ready-to-eat meals make up the rest of the FPI but they sit outside what a Friday-morning supermarket capture can measure honestly, so we leave them out. The methodology stays clean and the Index stays comparable week to week.
7. Cadence
Captured Friday morning, New Zealand time, from the public online catalogues of PAK’nSAVE and Woolworths. Published Sunday morning. The same Friday snapshot drives every household-size cell, so the figures across the table are self-consistent. Older snapshots feed the trend chart on the Index page.
8. Known limitations
- The basket starts at 9 staples and grows as Kepie Watch adds more. Week-to-week the Index is a like-for-like comparison; over months it reflects a steadily widening picture.
- We read online prices. In-store specials, clearance markdowns, regional pricing at some PAK’nSAVE locations, and Friday-only promotions may differ.
- Snapshots are taken once a week. The Index is a Friday number, not a continuous feed.
- We do not include restaurants, takeaways or ready-to-eat meals, alcohol, or delivery.
- Year-on-year change becomes available once the published series is twelve months deep. Until then the Index reports week-on-week and four-week change only.
9. Where the numbers come from
Every figure in the Index is built from the public online catalogues of PAK’nSAVE and Woolworths, read once a week. No supermarket sponsors Kepie Watch and no supermarket sees or influences what we publish. New World comes later this year, on the same terms.