# Kolmogorov-Smirnov test

2020 Mathematics Subject Classification: *Primary:* 62G10 [MSN][ZBL]

A non-parametric test used for testing a hypothesis $ H _ {0} $, according to which independent random variables $ X _ {1} \dots X _ {n} $ have a given continuous distribution function $ F $, against the one-sided alternative $ H _ {1} ^ {+} $: $ \sup _ {| x|<\infty } ( {\mathsf E} F _ {n} ( x) - F ( x) ) > 0 $, where $ {\mathsf E} F _ {n} $ is the mathematical expectation of the empirical distribution function $ F _ {n} $. The Kolmogorov–Smirnov test is constructed from the statistic

$$ D _ {n} ^ {+} = \ \sup _ {| x | < \infty } \ ( F _ {n} ( x) - F ( x) ) = \ \max _ {1 \leq m \leq n } \ \left ( \frac{m}{n} - F ( X _ {(} m) ) \right ) , $$

where $ X _ {(} 1) \leq \dots \leq X _ {(} n) $ is the variational series (or set of order statistics) obtained from the sample $ X _ {1} \dots X _ {n} $. Thus, the Kolmogorov–Smirnov test is a variant of the Kolmogorov test for testing the hypothesis $ H _ {0} $ against a one-sided alternative $ H _ {1} ^ {+} $. By studying the distribution of the statistic $ D _ {n} ^ {+} $, N.V. Smirnov [1] showed that

$$ \tag{1 } {\mathsf P} \{ D _ {n} ^ {+} \geq \lambda \} = $$

$$ = \ \sum_{k=0}^ { [ n ( 1 - \lambda ) ]} \lambda \left ( \begin{array}{c} n \\ k \end{array} \right ) \left ( \lambda + \frac{k}{n} \right ) ^ {k-} 1 \left ( 1 - \lambda - \frac{k}{n} \right ) ^ {n-} k , $$

where $ 0 < \lambda < 1 $ and $ [ a ] $ is the integer part of the number $ a $. Smirnov obtained in addition to the exact distribution (1) of $ D _ {n} $ its limit distribution, namely: If $ n \rightarrow \infty $ and $ 0 < \lambda _ {0} < \lambda = O ( n ^ {1/6} ) $, then

$$ {\mathsf P} \{ D _ {n} ^ {+} \geq \lambda \} = \ e ^ {- 2 \lambda ^ {2} } \left [ 1 + O \left ( \frac{1}{\sqrt n} \right ) \right ] , $$

where $ \lambda _ {0} $ is any positive number. By means of the technique of asymptotic Pearson transformation it has been proved [2] that if $ n \rightarrow \infty $ and $ 0 < \lambda _ {0} < \lambda = O ( n ^ {1/3} ) $, then

$$ \tag{2 } {\mathsf P} \left \{ \frac{1}{18n} ( 6 n D _ {n} ^ {+} + 1 ) ^ {2} \geq \lambda \right \} = e ^ {- \lambda } \left [ 1 + O \left ( \frac{1}{n} \right ) \right ] . $$

According to the Kolmogorov–Smirnov test, the hypothesis $ H _ {0} $ must be rejected with significance level $ \alpha $ whenever

$$ \mathop{\rm exp} \ \left [ \frac{( - 6 n D _ {n} ^ {+} + 1 ) ^ {2} }{18n} \right ] \leq \alpha , $$

where, by virtue of (2),

$$ {\mathsf P} \left \{ \mathop{\rm exp} \ \left [ \frac{( - 6 n D _ {n} ^ {+} + 1 ) ^ {2} }{18n} \right ] \leq \alpha \right \} = \alpha \left ( 1 + O \left ( \frac{1}{n} \right ) \ \right ) . $$

The testing of $ H _ {0} $ against the alternative $ H _ {1} ^ {-} $: $ \inf _ {| x | < \infty } ( {\mathsf E} F _ {n} ( x) - F ( x) ) < 0 $ is dealt with similarly. In this case the statistic of the Kolmogorov–Smirnov test is the random variable

$$ D _ {n} ^ {-} = - \inf _ {| x | < \infty } \ ( F _ {n} ( x) - F ( x) ) = \ \max _ {1 \leq m \leq n } \ \left ( F ( X _ {(} m) ) - m- \frac{1}{n} \right ) , $$

whose distribution is the same as that of the statistic $ D _ {n} ^ {+} $ when $ H _ {0} $ is true.

#### References

[1] | N.V. Smirnov, "Approximate distribution laws for random variables, constructed from empirical data" Uspekhi Mat. Nauk , 10 (1944) pp. 179–206 (In Russian) |

[2] | L.N. Bol'shev, "Asymptotically Pearson transformations" Theor. Probab. Appl. , 8 (1963) pp. 121–146 Teor. Veroyatnost. i Primenen. , 8 : 2 (1963) pp. 129–155 |

[3] | L.N. Bol'shev, N.V. Smirnov, "Tables of mathematical statistics" , Libr. math. tables , 46 , Nauka (1983) (In Russian) (Processed by L.S. Bark and E.S. Kedrova) |

[4] | B.L. van der Waerden, "Mathematische Statistik" , Springer (1957) |

#### Comments

There is also a two-sample Kolmogorov–Smirnov test, cf. the editorial comments to Kolmogorov test and, for details, [a1], [a2].

#### References

[a1] | G.E. Noether, "A brief survey of nonparametric statistics" R.V. Hogg (ed.) , Studies in statistics , Math. Assoc. Amer. (1978) pp. 39–65 |

[a2] | M. Hollander, D.A. Wolfe, "Nonparametric statistical methods" , Wiley (1973) |

**How to Cite This Entry:**

Kolmogorov-Smirnov test.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Kolmogorov-Smirnov_test&oldid=55016