How wrong, and in which direction
Size is the easy half. Direction is the half that repeats, and the half nobody measures.
Measuring the Error at All
Most forecasting functions do not keep an error record. What one contains, why it has to be built at the time, and what becomes possible once it exists.
Percentage Error and What It Hides
The standard accuracy measures each hide something different. Which to use for which decision, and why a single headline accuracy figure is usually misleading.
Direction: the Half Nobody Reads
A forecast that is wrong in the same direction every period is a correctable problem that mean absolute error cannot see. How to find it and what it usually means.
Bias, and the Correction That Fixes It
Once a forecast is shown to be biased, there are four possible fixes and only one of them is adding a number. How to choose, and how to record the choice.
What Good Looks Like at Each Horizon
There is no single standard for forecast accuracy. What error is achievable at each distance, where the floor comes from, and how to set a target that means something.
The Band, and Publishing It
A point estimate published alone acquires a precision it never had. Where the band comes from, how wide it should be, and how to publish it so it survives.
Comparing Against Doing Nothing
Every forecast should be measured against the cheapest alternative to having one. The three naive benchmarks, and what it means when the forecast loses.