| 释义 |
sigma A Greek letter, written as ∑ in upper case, or σ in lower case. This is used in economics in various ways. Upper case ∑ is used to denote the sum of the items following it; thus 0198607679.sigma.1.jpg Here the subscript 1 and superscript N after ∑ denote the limits of the values i should take. In cases where the limits of the summation are obvious, σi may be used. Lower case σ is used to denote the standard deviation, a widely used measure of dispersion, defined as the square root of the mean sum of squares of the distances between each xi and their mean μx. Thus 0198607679.sigma.2.jpg (this is true if μx is already known; if μx is itself being calculated from the same data set, 0198607679.sigma.3.jpg gives an unbiased estimate). The symbols ∑ and σ are also both frequently used in economics in other senses defined ad hoc. |