Fan Chart
Updated 2026-08-15
INTRODUCTION
English translation pending.
CORE DEFINITION
A fan chart displays a forecast as a central projection surrounded by bands that widen as the horizon extends, often marking a fifty and a ninety percent interval. The core proposition is that a point forecast misrepresents the future by hiding the fact that uncertainty accumulates with time, while a band whose width is calibrated makes the honest spread visible. It was widely popularised by the Bank of England's inflation reports. The key qualification is that the bands are only as trustworthy as the model and the distributional assumptions behind them; a fan conveys false precision when structural breaks lie outside the assumed range.
SCAFFOLDING EFFECT
Reduce cognitive load
- Show The Interval: publish the band, not only the central line, so readers see the spread. - Let It Widen: draw bands that grow with horizon instead of a constant error margin. - Talk In Ranges: phrase commitments in intervals rather than single point predictions.
Anchor fast decisions
Forecast errors accumulate rather than cancelling, because each step forward compounds the uncertainty carried from the previous one, so the variance of the prediction grows with the horizon. Drawing that growth as a widening band maps the accumulation onto geometry: near-term certainty and far-term doubt appear as shape rather than prose. Readers then calibrate their own trust correctly, preparing for the spread instead of anchoring on a line that will almost surely be missed.
MINIMUM ACTION
In progress 0/1Practice this model in one real situation:
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Source support: Explicit
- en.wikipedia.orghttps://en.wikipedia.org/wiki/Fan_chart_(statisticsverified
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