Forecast Range¶

Projection method — instantiates Uncertainty Explicitness

Communicates a future estimate as a range or a small set of scenarios rather than one point number — carrying the assumptions the range depends on and the triggers that mark when it has gone stale — so nobody plans against a single guess about an unknowable future.

A Forecast Range represents uncertainty about the future specifically: instead of a single projected number, it gives a range or a small envelope of scenarios (low / expected / high), each resting on stated assumptions. Its distinguishing move among its siblings is that its uncertainty is largely irreducible and time-bound — it comes not from a sample you could enlarge but from conditions that have not happened yet — so the range must travel with the assumptions that generate it and the triggers that declare it obsolete. Unlike a confidence interval, which narrows as you collect more data, a forecast range widens the further out it reaches and can be invalidated overnight by a changed condition, which is why the assumptions and the update rule are part of the mechanism, not an afterthought.

Example¶

A retailer forecasting next year's demand for a seasonal product resists the single number the planning template wants. Instead it publishes a range shaped like a fan: near-term months are tight, far-out months flare wide, and the whole envelope rests on three named assumptions — a stable promotional calendar, no major competitor entry, and input costs within a stated band. When a competitor announces a rival launch mid-year, a pre-set trigger fires ("competitor entry in category") and the forecast is re-run rather than trusted; the planners had agreed in advance that this event, not the calendar, is what makes the old range stale. The range also changes how the number is used downstream: inventory is planned against the band's width, not a point, so the buffer for the wide far-out months is deliberate rather than an accident of a single guess.^[1]

How it works¶

The method produces a distribution over futures rather than a value: a range, quantile band, or scenario set, wider where the future is less constrained. What distinguishes it from the sampling-based siblings is that its width is driven by assumptions about conditions, not by sample size — so the mechanism explicitly logs those assumptions alongside the range, and pairs the range with update triggers: the events, dates, or threshold crossings that should force a re-forecast. A forecast range without its assumptions and its expiry conditions is just a wider guess that will quietly go stale.

Tuning parameters¶

Range form — a symmetric interval, an asymmetric fan, or a discrete scenario set (low/base/high). Scenarios communicate distinct futures; continuous bands communicate graded likelihood.
Horizon and flare — how far out the forecast reaches and how fast the band widens; too-narrow far-out bands are the signature dishonest forecast.
Scenario count — two or three named scenarios versus many; more scenarios cover the space but dilute attention and invite false discrimination between near-identical futures.
Assumption visibility — whether the conditions generating the range are published with it or buried; hidden assumptions turn an honest range into an unfalsifiable one.
Update-trigger sensitivity — how large a condition change fires a re-forecast; too twitchy and the range never settles, too dull and it outlives its assumptions.

When it helps, and when it misleads¶

Its strength is that it refuses the false comfort of a single future and forces planning against a width — buffers, hedges, and contingencies attach to the range, not to a point that was always going to be wrong. Its failure modes are specific to forecasting. A range can be drawn too narrow to look competent, hiding real tail risk behind a tidy band. It can outlive its assumptions when nobody watches the triggers, so a stale range keeps steering decisions after the world moved. And the far-out end can imply the future is bounded when it is genuinely open — a fan chart still cannot show the scenario nobody modelled. The discipline that guards against this is publishing the assumptions with the range, honestly flaring the band with the horizon, and enforcing the update triggers so an obsolete forecast is retired rather than trusted.

How it implements the components¶

confidence_representation — the range or scenario envelope is the confidence representation for a future quantity: it shows the spread of plausible outcomes rather than a single value.
assumption_log — because the width is generated by conditions rather than a sample, the mechanism carries the explicit assumptions the range depends on.
update_trigger — it pairs the range with the events, dates, or threshold crossings that mark it stale and force a re-forecast.

It does not itself grade how good the input evidence is (evidence_quality_trace) — that is Evidence Grade Rubric — and it does not render the band as a shaded visual (visual_uncertainty_encoding), which is what Uncertainty Band does with a forecast range's output.

Instantiates: Uncertainty Explicitness — it keeps the uncertainty of a future estimate visible and its expiry explicit.
Consumes: Assumption Register can hold the conditions each scenario depends on.
Sibling mechanisms: Uncertainty Band · Assumption Register · Confidence Interval · Probability Estimate · Caveated Decision Memo · Confidence Label · Error Bar · Evidence Grade Rubric · Known Unknowns Log · Model Limitations Card · Risk Register

Notes¶

A forecast range and a confidence interval both report a spread, but the uncertainty is different in kind: an interval's shrinks as you gather more data; a range's is dominated by conditions that have not occurred and can widen as the horizon lengthens or an assumption fails. Treating a forecast range like a confidence interval — assuming more data will tighten it — is a common and costly category error.

References¶

[1] A fan chart displays a central projection surrounded by shaded bands that widen with the forecast horizon, so the growing uncertainty of the far future is shown rather than implied — the visual form most associated with honest range forecasting in economics and demand planning. ↩