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SDCalc
入門基礎知識·10 min

認識變異數:標準差的基石

掌握變異數的概念及其與標準差的關係。學習公式、計算方法,以及變異數在統計學中的實際應用。

什麼是變異數?

變異數衡量一組數字相對於其平均值的分散程度。它是每個數值與平均數之差的平方的平均值——也是標準差這個概念的基礎。

每個長條代表與平均數的偏差平方。變異數 = 這些長條的平均值。

變異數公式

母體變異數

σ² = Σ(xᵢ - μ)² / N

樣本變異數

s² = Σ(xᵢ - x̄)² / (n-1)
1

計算平均數

將所有數值加總,除以個數。
2

計算每個偏差

用每個資料點減去平均數。
3

將偏差平方

這會消除負值,並強調較大的偏差。
4

計算偏差平方的平均值

除以 N(母體)或 n-1(樣本)。

為什麼要平方偏差?

三個關鍵原因

1. 消除負值: 如果不平方,正偏差和負偏差會互相抵消,加總後為零。 2. 加重離群值的影響: 平方會讓偏離平均數較遠的值獲得更大的權重。 3. 數學特性: 變異數具有在統計推論中非常實用的代數性質。

範例:為什麼不直接用絕對值?

資料集:2, 4, 4, 4, 5, 5, 7, 9(平均數 = 5) 平均絕對偏差: |2-5| + |4-5| + ... = 14 MAD = 14/8 = 1.75 變異數(平方): (2-5)² + (4-5)² + ... = 32 Var = 32/8 = 4

變異數 vs 標準差

兩者的關係

Standard Deviation = √Variance → σ = √σ²

變異數 (σ²)

- 單位是平方(例如 cm²、$²) - 較難直接解讀 - 適合數學運算 - 獨立變數可直接相加

標準差 (σ)

- 與原始資料相同單位 - 較容易解讀 - 適合溝通報告 - 用於 Z 分數和信賴區間

變異數的應用

雖然標準差更常被報告使用,但變異數有其特定的用途:

  • 變異數分析 (ANOVA):跨組別比較平均數
  • 投資組合理論:報酬率的變異數用於最佳化配置
  • 迴歸分析:R² 即為已解釋變異數除以總變異數
  • 主成分分析 (PCA):最大化已解釋的變異數

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.