What a price history shows that a percentage cannot

Three months is the window the regulator works in

The Advertising Standards Authority expects advertisers to provide at least three months of pricing history, including information about how long the product was sold at any intervening prices, in order to assess the usual selling price of a product. In the absence of such information, the ASA may consider a savings claim to be misleading.

Three months is therefore a useful window for a buyer as well. It is long enough that a short promotional period cannot dominate it and short enough that the item is still the same item. An offer that looks good against yesterday's price and poor against the last 12 weeks is an offer you can see through.

The ASA adds two conditions that are worth borrowing directly. The higher reference price should generally have been charged for a longer duration than the promotional price. And the marketer should be able to show either that a significant number of sales were made at the reference price, or that the higher price was a realistic selling price.

1. Set a window of 3 months and record the price on the same weekday each week, which gives about 13 observations
2. Record the delivered total, not the headline, so that the series stays comparable
3. Note the date beside every figure; a price without a date is not evidence
4. Count how many weeks the highest price was actually charged
5. Compare today's price with the weighted average of the window, not with the highest number in it
6. Keep the record for the item you want, not for the category

Weight the price by the days it was charged

Here is the calculation that a percentage hides. Take a 16-week window in which an example item sold at £50 for 10 weeks and £35 for 6 weeks. The time-weighted average is £50 times 10 plus £35 times 6, all divided by 16: that is £500 plus £210, which is £710, divided by 16, giving £44.38.

Against that average, buying at £35 saves £9.38, not the £15 implied by "was £50". The £50 was the price for 10 of the 16 weeks, which is 62.5% of the window, so it is a defensible reference price and still not the number that describes what the item normally costs.

Now change one input. If the same item sold at £50 for only 2 weeks and £35 for 14 weeks, the weighted average is £100 plus £490, which is £590, divided by 16, giving £36.88. The saving against the usual price collapses to £1.88, and the £50 was charged for 12.5% of the window: the ASA's duration test would be hard to satisfy.

A year-old reference price is the wrong comparison

Prices drift upward even when nothing about the product changes. The Office for National Statistics put the CPI annual inflation rate at 3.5% for September 2026 in the bulletin released on 21 October 2026. On that rate, a £50 price from twelve months ago corresponds to about £51.75 in today's money.

That cuts both ways. A reference price taken from last year understates today's usual price by roughly the inflation rate, which makes a current reduction look smaller than it is. A reference price that was set last year and never charged since overstates it, because the item may have sat below that figure for the whole period.

The practical rule is the same one the ASA applies: recency. A price history is useful because it is recent and continuous. A single remembered price from a year ago is neither, and it is the commonest reason a shopper is certain an offer is good.

Keeping a record that is worth keeping

A usable price history takes about 2 minutes a week. Pick the item, pick a weekday, record the delivered total and the date. Twelve weeks of that gives you 12 observations, which is enough to see whether a price moves in a cycle, drifts, or sits flat and drops once a quarter.

Record the delivered total rather than the headline, because delivery is the line that moves most between sellers and most often between weeks. A £35 item with £4.99 delivery and a £35 item collected in store are not the same observation, and a series that mixes them is not a series.

Set an alert threshold before you start watching rather than after. Deciding in advance that you will buy at £32 or below removes the decision from the moment when a countdown is running. The price-alert planner on this site takes your threshold and your window so that the number is fixed before the pressure arrives.

What a price history still cannot tell you

A series of prices for one listing does not tell you whether the listing stayed the same product. Model refreshes, changed bundle contents and different pack sizes all show up as a price movement when they are really a different item, which is why the item code matters as much as the number.

It also says nothing about availability. A low price on a listing that is out of stock for 6 weeks is not a price anybody paid, and it will pull a weighted average down in a way that flatters the seller's history rather than describing your options.

Finally, it cannot anticipate a genuine change in cost. A supply shock or a currency move resets the level, and a window drawn from before the reset describes a market that no longer exists. When the whole category steps up at once, the old average is history rather than a target.

Before you buy

• Choose a 3-month window and a fixed weekday for each observation
• Record the delivered total and the date, never the headline alone
• Work out the time-weighted average rather than using the highest figure you saw
• Count how many weeks the highest price was actually charged before treating it as usual
• Treat any reference price older than a year as stale, and adjust it for inflation if you use it
• Fix your buying threshold in advance and write it down
• Check the item code and pack size each time so the series stays comparable

Questions readers ask

How long should I track a price before buying?

Three months matches the window the ASA uses to assess a usual selling price, and about 13 weekly observations is enough to see a pattern. If the purchase is urgent, four weeks still beats a single comparison, because it tells you whether today's figure is the lowest you have seen or simply the first.

Should I use the lowest price I have seen as my target?

Use it as the floor and the weighted average as the benchmark. The lowest observation may have been a brief error or an out-of-stock listing. Buying at or below the time-weighted average of your window means paying less than the item usually costs, which is a target you can hit repeatedly rather than once.

Does a long price history make a seller trustworthy?

It makes a specific claim checkable, which is different. The ASA's position is that a savings claim needs pricing history behind it, including how long any intervening prices applied. A seller with a stable, visible history is easier to verify; a seller whose history you cannot see is not dishonest by default, only unverifiable.

Why did my recorded price jump between two weeks?

Check the three usual causes before concluding the price moved. The listing may have changed pack size or bundle contents; delivery may have shifted between free and charged; or you may have compared a marketplace seller with the retailer's own listing. Recording the delivered total and the item code removes all three from the series.

Sources

1. Advertising Standards Authority — Promotional savings claims (at least three months of pricing history, duration and sales tests) — checked 2026-11-03
2. Office for National Statistics — Inflation and price indices (CPI annual rate 3.5%, September 2026, released 21 October 2026) — checked 2026-11-03
3. GOV.UK — Unfair commercial practices (CMA207), the CMA's guidance on the unfair trading provisions of the DMCC Act 2024 — checked 2026-11-03