McKay's approximation for the coefficient of variation

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In statistics, McKay's approximation of the coefficient of variation is a statistic based on a sample from a normally distributed population. It was introduced in 1932 by A. T. McKay.[1] Statistical methods for the coefficient of variation often utilizes McKay's approximation.[2][3][4][5]

Let  x_i , i = 1, 2,\ldots, n be  n independent observations from a  N(\mu, \sigma^2) normal distribution. The population coefficient of variation is  c_v = \sigma / \mu . Let  \bar{x} and  s \, denote the sample mean and the sample standard deviation, respectively. Then  \hat{c}_v = s/\bar{x} is the sample coefficient of variation. McKay’s approximation is

K = \left( 1 + \frac{1}{c_v^2} \right) \ \frac{(n - 1) \ \hat{c}_v^2}{1 + (n - 1) \ \hat{c}_v^2/n}

Note that in this expression, the first factor includes the population coefficient of variation, which is usually unknown. When  c_v is smaller than 1/3, then  K is approximately chi-square distributed with  n - 1 degrees of freedom. In the original article by McKay, the expression for  K looks slightly different, since McKay defined  \sigma^2 with denominator  n instead of  n - 1 . McKay's approximation,  K , for the coefficient of variation is approximately chi-square distributed, but exactly noncentral beta distributed .[6]


  1. ^ McKay, A. T. (1932) Distribution of the coefficient of variation and the extended “t” distribution. Journal of the Royal Statistical Society 95, 695–698
  2. ^ Iglevicz, Boris; Myers, Raymond (1970). "Comparisons of approximations to the percentage points of the sample coefficient of variation". Technometrics 12 (1): 166–169. JSTOR 1267363. 
  3. ^ Bennett, B. M. (1976). On an approximate test for homogeneity of coefficients of variation. In: Ziegler, W. J. (ed.) Contributions to Applied Statistics Dedicated to A. Linder. Experentia Suppl. 22, 169-171
  4. ^ Vangel, Mark G. (1996). "Confidence intervals for a normal coefficient of variation". The American Statistician 50 (1): 21–26. doi:10.1080/00031305.1996.10473537. JSTOR 2685039. .
  5. ^ Forkman, Johannes. "Estimator and tests for common coefficients of variation in normal distributions". Communications in Statistics - Theory and Methods 38. pp. 21–26. Retrieved 2013-09-23. 
  6. ^ Forkman, Johannes; Verrill, Steve. "The distribution of McKay's approximation for the coefficient of variation". Statistics & Probability Letters 78. pp. 10–14. doi:10.1016/j.spl.2007.04.018. Retrieved 2013-09-23.