單字庫：最有效的背單字遊戲

Rank: we take the app's best Top Grossing rank on the latest chart day. Category ranks are converted into an equivalent overall rank, because a #10 in Finance earns far less than a #10 overall.

Curve: that equivalent rank is read off a rank-to-revenue curve anchored on public company revenue and market-data reports for the US iPhone App Store.

Country: the curve is scaled per country (Japan is roughly a third of the US, Türkiye under 1%). Where we have no scale, such as China, there is no estimate.

Range: the likely range widens with country, category and chart depth. It is a plausible span, not a statistical confidence interval.

Not included: ads, payments outside the App Store, and any difference between gross sales and what the developer keeps. Low confidence means indirect data; medium means several independent sources agree.