Fable
Twenty rains kept in an iron box
The Rain Master Academy trained young people to judge weather patterns. On ordinary days they watched clouds, felt the wind and listened to thunder, each with their own insights. But when it came to evaluations, everyone claimed their judgment was accurate, and the arguments never ended.
The curator initially had everyone casually compare the most recent rains. Some picked the spring rain they were familiar with, some picked violent summer rain, and some avoided the ugly fog rain. As a result, everyone could prove they were not bad.
For the first patch, the curator collected over a hundred weather records, the more the better. The apprentices read until their heads spun, yet the comparison was still chaotic, because among them were rains with incomplete records and rains whose results had later been rewritten by someone.
An old cloud watcher took out an iron box containing only twenty rains. Each had that day's cloud shape, wind direction, river surface changes, final rainfall, and the accepted answer. They were few, but covered common, rare, easily misjudged, and high-cost situations.
The young people thought twenty sessions were too few. The old cloud watchers had them first answer on these twenty, and many seemingly impressive sayings were immediately exposed: they could guess a downpour but always missed drizzle; they could read spring but misjudged autumn fog.
The academy would still collect new weather later, but the rain in the iron box was not changed lightly. To add a storm, it had to be explained which type of gap it filled, whether the old answers were reliable, and whether it would affect comparisons across the years.
From then on, everyone had a common ruler. The iron box was not all the rain in the world, but it was a sufficiently credible and sufficiently representative testing ground, so that progress and regression could both be seen clearly.
Later, an apprentice wanted to turn the iron box into a treasure chest and stuff in every kind of weather. The curator stopped him and said the iron box was valuable for being trustworthy and stable, not for an intimidating quantity. It had to sufficiently represent key challenges and also be clean enough that this year's judgment could be compared with last year's. Once the common ruler drifts, progress becomes impossible to discuss. The apprentices later became more convinced, because winning or losing no longer depended on who picked rain that favored themselves. Everyone faced the same set of credible weather, and mistakes were obvious. The iron box brought arguments back from eloquence to evidence. Later, every time the academy claimed progress, it first had to pass through the rain in the iron box. This sentence was later written into the newcomers' handbook, reminding everyone to look at the on-site cost first before talking about saving effort. And because of this reminder, when similar difficulties arose later, people would not rush to apply old methods.