How we measure PC prices, and why the average listing price lies

The first version of a price index anyone writes is: average today's listing prices, compare to last month's average. One line of SQL. We wrote that version too, and the number it produced moved around convincingly. It just wasn't measuring prices. It was measuring which machines happened to be listed that day.

Catalogs churn constantly. A brand launches a run of budget laptops, a workstation line sells out, a storefront reshuffles what's in stock. None of that changes the price of any computer, and all of it moves a listing average, often by more than actual price movement does. Retire a few expensive machines at the right moment and the average "crashes" in a month where everything that stayed on the shelf got more expensive.

Inflation statisticians dealt with this decades ago, and we do the standard thing: follow the same machine over time. Each product contributes the ratio of its price now to its own price at the start of the window, and the published figure for a category is the median of those per-product ratios. A machine has to be tracked at both ends of the window to count at all. Fifty new budget laptops contribute nothing, because they have no before. A retired workstation contributes nothing either.

Median rather than mean, because configure-to-order machines occasionally re-anchor which configuration counts as the base model, and that shows up as a four-figure percent change on one row without anything getting cheaper or dearer. A mean would pass that straight through to the headline. And when a category has too few matched pairs in the window, we publish a blank rather than a number built on three machines.

Some rows get filtered out before any of this runs. Deposit and reservation entries priced at a few dollars, which would otherwise win every cheapest-machine cell they touch. Re-anchored configurations, per above. Out-of-stock listings, since a price nobody can transact at tells you nothing about the market. And the whole computation is USD-only, because mixing currencies would let an exchange-rate swing impersonate a price move.

We wrote this down because we want the figures to be checkable. Everything on the PC Price Index is computed exactly as described, from manufacturer-listed prices for complete systems, with its window, sample size and as-of date printed beside it. Cite it with a link. If you think the method is wrong somewhere, tell us.