No. 240 · Evidence · Statistics and Probability
Variance
Every process wobbles; the skill is telling ordinary wobble from a real warning sign.
In the Almanac
“Shewhart’s control chart”, 1924 · “JR Central”, 2010s
Both stories in full, three ways to use it and where it misleads.
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Questions
What is Variance?
Variance measures how much results swing around their average. In 1924 Walter Shewhart drew the first control chart to separate everyday common-cause wobble from real, assignable problems. JR Central keeps average Shinkansen delays under a minute by attacking variation everywhere.
How do you use Variance?
Learn the natural range of your numbers before reacting, smooth irregular spending into steady monthly amounts, and keep routines consistent so real problems stand out. Just leave room for the healthy variation that creativity needs.
When does Variance matter?
Variance is one of the 68 mental models for evidence, the situations where you ask: Is this really true? Within that, it belongs to “Reading the numbers”, alongside Base Rate Fallacy, Bayes’ Theorem and Central Limit Theorem.
What is an example of Variance?
Two stories show it at work: Shewhart’s control chart (1924) and JR Central (2010s). Both are told in full in The Mental Models Daily Almanac, which has two stories for each of its 365 models.
Which mental models are related to Variance?
Standard Deviation, Feedback Loops and Homeostasis. Variance itself comes from statistics and probability.