Sigma (σ) in HTML
Sigma is the eighteenth Greek letter, written Σ and σ. The capital is at U+03A3, the small letter at U+03C3, and a separate final form exists at U+03C2.
At a glance
- Character
- Σ σ
- Named entity
- Σ σ
- Decimal reference
- Σ σ
- Hex reference
- Σ σ
- Unicode code point
- U+03A3, U+03C3
- Unicode name
- GREEK CAPITAL LETTER SIGMA, GREEK SMALL LETTER SIGMA
- In the entity reference
- Greek
In detail
Sigma is the only letter in the Greek alphabet with two lowercase forms. The ordinary small sigma is used everywhere except at the end of a word, where the final sigma at U+03C2 is used instead. That is a spelling rule rather than a style choice, so Greek text set entirely with the ordinary form is misspelled, and a naive lowercasing of Greek capitals produces exactly that error unless the implementation knows the rule.
The capital is the letter that people reach for when they want a summation sign, and it is the wrong character. The n-ary summation operator has its own code point at U+2211, drawn slightly larger and with a different weight balance because it has to carry limits above and below. Substituting the letter is invisible on screen and breaks every comparison, search and conversion afterwards.
In statistics the small sigma is the population standard deviation, with the capital sometimes used for a covariance matrix. As with mu, the population and sample quantities are deliberately distinguished, the sample standard deviation being written with an s, and reporting one while meaning the other misstates how much is known.
The six sigma name in process improvement comes from the same letter: a process held to plus or minus six standard deviations from the mean produces very few defects. That is a use of the concept rather than of the character, and the name is always spelled out in Latin letters.
Examples
<p>μ = 100, σ = 15</p>
<p>About 95% of values fall within μ ± 2σ.</p><p>Greek capital sigma: Σ (U+03A3)</p>
<p>summation operator: ∑ (U+2211)</p>
<p>final sigma: ς (U+03C2)</p>Things that catch people out
- Using the Greek capital as a summation sign produces a formula that no search or conversion will treat as mathematics.
- Lowercasing Greek text without handling the final sigma rule misspells every word that ends in the letter.
- The population and sample standard deviations use different symbols and different divisors. The distinction is not cosmetic.
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