Personal Inflation: Your Own Basket Index vs the Official Rate

Every month a single inflation figure is published, and every month a lot of people look at it and think: that is not what my shopping feels like. They are often right. The official rate describes an average household. Nobody’s shopping is exactly average.

Why your inflation differs from the official figure

An official consumer price index measures the price of a basket of goods and services that represents spending across the whole population. It includes rent, transport, energy, clothing, restaurants, and much more, each weighted by how much households spend on it on average.

Your own experience can differ for simple reasons:

• You buy different things. If coffee, olive oil and baby food are a big part of your shopping, their prices matter far more to you than to the average.
• You spend in different shares. A household that spends half its grocery money on fresh produce feels produce prices much more than one that spends a fifth.
• You shop at different places. The same product can move differently at different chains.
• You only see part of it. At the till you see groceries and household goods, not everything an official index covers.

None of this makes the official number wrong. It answers a different question.

What a personal basket index measures

A personal basket index answers: how much have the prices of the things I actually buy changed, in the proportions I actually buy them? It is built from your own receipts, so it is your figure, not an official inflation rate, and it only covers what your receipts cover.

How Tasarruf computes it, in plain words

The method is designed to be honest about what it knows. In steps:

1. Only matched products. A price change only means something if it is the same product at both points in time. Only products that are confirmed as the same, or matched with high confidence, are used. See product matching.
2. Monthly unit values. For each product, the unit price (per kg, per litre or per piece) paid in a month is taken as its monthly unit value, and those values are compared over a 3-month window.
3. Price relatives per product. Each product’s unit value is compared with its earlier value, giving a ratio: 1.10 means 10 % more expensive.
4. Geometric mean per category. Within a category, the ratios are combined with a geometric mean (this is called a Jevons index). A doubling and a halving cancel out, and one expensive item cannot drag the result around.
5. Your own weights. Categories are weighted by your own spending shares over the previous 12 months. If dairy was 15 % of your spending, dairy counts for 15 %.
6. Chained yearly. Weights are updated once a year, and the index is linked across those updates so the series stays continuous as your habits change.

A small example for one category with three matched products:

Product	Before	Now	Ratio
Milk, per l	1.20	1.32	1.10
Yoghurt, per kg	4.00	4.20	1.05
Cheese, per kg	12.00	11.40	0.95

The geometric mean of 1.10, 1.05 and 0.95 is about 1.031, so dairy rose about 3.1 % for you. That category result is then weighted by your dairy share alongside your other categories.

Coverage, always shown

An index built from receipts can only see what you bought, recorded and matched. So Tasarruf always shows coverage: how much of your spending is represented by the matched products in the index.

If a large part of your spending is on products bought only once, or not matched yet, the figure rests on a small slice of your shopping. Coverage tells you how much to trust it.

Why it sometimes says “not enough data”

When coverage is below 60 %, or fewer than 3 matched items are available, Tasarruf does not show a number. It says “not enough data” instead.

That can feel unhelpful, but a number built on two products would look precise and mean almost nothing. Showing nothing is better than showing a misleading percentage. Usually it fills in on its own after a few weeks of regular scanning, as the same products appear on several receipts and get matched.

Getting a useful index sooner

• Scan every grocery receipt, not just the big ones. Routine purchases are what make the index work.
• Answer the “Is this the same product?” questions. Confirmed matches count.
• Import online shopping invoices too. See importing e-Arşiv PDFs.

How Tasarruf helps

Tasarruf builds your personal basket index from your own receipts, shows its coverage every time, and refuses to show a number when the data is too thin. Alongside it you get the price history of each product and your monthly spending by category, all from the same scans. Learn more about Tasarruf.