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In statistics, dependence is any statistical relationship between two random variables or two sets of data. Correlation refers to any of a broad class of statistical relationships involving dependence. Familiar examples of dependent phenomena include the correlation between the physical statures of parents and their offspring, and the correlation between the demand for a product and its price. Correlations are useful because they can indicate a predictive relationship that can be exploited in practice. For example, an electrical utility may produce less power on a mild day based on the correlation between electricity demand and weather. In this example there is a causal relationship, because extreme weather causes people to use more electricity for heating or cooling; however, statistical dependence is not sufficient to demonstrate the presence of such a causal relationship (i.e., correlation does not imply causation). Formally, ''dependence'' refers to any situation in which random variables do not satisfy a mathematical condition of probabilistic independence. In loose usage, ''correlation'' can refer to any departure of two or more random variables from independence, but technically it refers to any of several more specialized types of relationship between mean values. There are several correlation coefficients, often denoted ''ρ'' or ''r'', measuring the degree of correlation. The most common of these is the Pearson correlation coefficient, which is sensitive only to a linear relationship between two variables (which may exist even if one is a nonlinear function of the other). Other correlation coefficients have been developed to be more robust than the Pearson correlation – that is, more sensitive to nonlinear relationships.〔Croxton, Frederick Emory; Cowden, Dudley Johnstone; Klein, Sidney (1968) ''Applied General Statistics'', Pitman. ISBN 9780273403159 (page 625)〕〔Dietrich, Cornelius Frank (1991) ''Uncertainty, Calibration and Probability: The Statistics of Scientific and Industrial Measurement'' 2nd Edition, A. Higler. ISBN 9780750300605 (Page 331)〕〔Aitken, Alexander Craig (1957) ''Statistical Mathematics'' 8th Edition. Oliver & Boyd. ISBN 9780050013007 (Page 95)〕 Mutual information can also be applied to measure dependence between two variables. ==Pearson's product-moment coefficient== (詳細はPearson product-moment correlation coefficient, or "Pearson's correlation coefficient", commonly called simply "the correlation coefficient". It is obtained by dividing the covariance of the two variables by the product of their standard deviations. Karl Pearson developed the coefficient from a similar but slightly different idea by Francis Galton. The population correlation coefficient ρ''X,Y'' between two random variables ''X'' and ''Y'' with expected values μ''X'' and μ''Y'' and standard deviations σ''X'' and σ''Y'' is defined as: : where ''E'' is the expected value operator, ''cov'' means covariance, and ''corr'' is a widely used alternative notation for the correlation coefficient. The Pearson correlation is defined only if both of the standard deviations are finite and nonzero. It is a corollary of the Cauchy–Schwarz inequality that the correlation cannot exceed 1 in absolute value. The correlation coefficient is symmetric: corr(''X'',''Y'') = corr(''Y'',''X''). The Pearson correlation is +1 in the case of a perfect direct (increasing) linear relationship (correlation), −1 in the case of a perfect decreasing (inverse) linear relationship (anticorrelation),〔Dowdy, S. and Wearden, S. (1983). "Statistics for Research", Wiley. ISBN 0-471-08602-9 pp 230〕 and some value between −1 and 1 in all other cases, indicating the degree of linear dependence between the variables. As it approaches zero there is less of a relationship (closer to uncorrelated). The closer the coefficient is to either −1 or 1, the stronger the correlation between the variables. If the variables are independent, Pearson's correlation coefficient is 0, but the converse is not true because the correlation coefficient detects only linear dependencies between two variables. For example, suppose the random variable ''X'' is symmetrically distributed about zero, and ''Y'' = ''X''2. Then ''Y'' is completely determined by ''X'', so that ''X'' and ''Y'' are perfectly dependent, but their correlation is zero; they are uncorrelated. However, in the special case when ''X'' and ''Y'' are jointly normal, uncorrelatedness is equivalent to independence. If we have a series of ''n'' measurements of ''X'' and ''Y'' written as ''xi'' and ''yi'' where ''i'' = 1, 2, ..., ''n'', then the ''sample correlation coefficient'' can be used to estimate the population Pearson correlation ''r'' between ''X'' and ''Y''. The sample correlation coefficient is written : where x and y are the sample means of ''X'' and ''Y'', and ''s''''x'' and ''s''''y'' are the sample standard deviations of ''X'' and ''Y''. This can also be written as: : If ''x'' and ''y'' are results of measurements that contain measurement error, the realistic limits on the correlation coefficient are not −1 to +1 but a smaller range. For the case of a linear model with a single independent variable, the coefficient of determination (R squared) is the square of r, Pearson's product-moment coefficient . 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Correlation and dependence」の詳細全文を読む スポンサード リンク
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