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SDCalc
入門核心概念·6 min

68-95-99.7 經驗法則完整說明

掌握常態分配的經驗法則(68-95-99.7 法則)。學習如何快速估算機率並利用標準差識別離群值。

什麼是經驗法則?

經驗法則(又稱 68-95-99.7 法則或三個標準差法則)是一個簡便的記憶方法,用來記住常態分配中,落在平均數 ±1、±2、±3 個標準差範圍內的數值百分比。

68%

在 ±1σ 範圍內

95%

在 ±2σ 範圍內

99.7%

在 ±3σ 範圍內

圖解說明

The Classic Bell Curve

範圍百分比
μ ± 1σ68.27%
μ ± 2σ95.45%
μ ± 3σ99.73%

實務應用

  • 快速估算機率:不需要複雜的計算,你就能估算出大約 95% 的資料落在平均數 ±2 個標準差的範圍內。
  • 離群值偵測:超過 3σ 之外的資料點出現的機率不到 0.3%,值得進一步調查。
  • 品質管控:六標準差方法論利用這個法則來設定品質閾值並識別製程變異。

計算範例

範例:學測成績

學測成績呈常態分配,μ = 1050,σ = 200。 - 68% 的成績落在 850 到 1250 之間(±1σ) - 95% 的成績落在 650 到 1450 之間(±2σ) - 99.7% 的成績落在 450 到 1650 之間(±3σ) 考到 1450 以上的學生,已經位於所有考生的前 ~2.5%。

使用限制

僅適用於常態分配

經驗法則只適用於遵循常態(高斯)分配的資料。對於偏態或非常態的資料,這些百分比並不適用。使用此法則前,請務必先確認你的資料是否為常態分配。

Further Reading

How to Read This Article

A statistics tutorial is a practical interpretation guide, not just a formula dump. It refers to the assumptions, notation, and reporting language that analysts need when they explain a result to a teacher, manager, client, or reviewer. The article body covers the specific topic, while the sections below create a common interpretation frame that readers can reuse across related metrics.

Reading goalWhat to focus onCommon mistake
DefinitionWhat the metric is and what quantity it summarizesTreating the formula as self-explanatory
Formula choiceSample versus population assumptions and notationUsing n when n-1 is required or vice versa
InterpretationWhether the result indicates concentration, spread, or riskCalling a large value good or bad without context

Frequently Asked Questions

How should I interpret a high standard deviation?

A high standard deviation means the observations are spread farther from the mean on average. Whether that spread is acceptable depends on the context: wide dispersion might signal risk in finance, instability in manufacturing, or genuine natural variation in scientific data.

Why do some articles mention n while others mention n-1?

The denominator reflects the difference between population and sample formulas. Population variance and population standard deviation use N because the full dataset is known. Sample variance and sample standard deviation often use n-1 because Bessel’s correction reduces bias when estimating population spread from a sample.

What is a statistical interpretation guide?

A statistical interpretation guide is a page that moves beyond arithmetic and explains meaning. It tells you what a metric is, when the formula applies, and how to describe the result in plain English without overstating certainty.

Can I cite this article in a report?

You should cite the underlying authoritative reference for formal work whenever possible. This page is best used as an explanatory bridge that helps you understand the concept before quoting the original standard or handbook.

Why include direct citations on every article page?

Direct citations give readers a route to verify the definition, notation, and assumptions. That improves trust and reduces the chance that a simplified explanation is mistaken for the entire technical standard.

Authoritative References

These sources define the concepts referenced most often across our articles. Bessel's correction is a sample adjustment, variance is a squared measure of spread, and standard deviation is the square root of variance expressed in the same units as the data.