DEFINITION of 'Correlation'
In the world of finance, a statistical measure of how two securities move in relation to each other. Correlations are used in advanced portfolio management.
INVESTOPEDIA EXPLAINS 'Correlation'
Correlation is computed into what is known as the correlation coefficient, which ranges between 1 and +1. Perfect positive correlation (a correlation coefficient of +1) implies that as one security moves, either up or down, the other security will move in lockstep, in the same direction. Alternatively, perfect negative correlation means that if one security moves in either direction the security that is perfectly negatively correlated will move in the opposite direction. If the correlation is 0, the movements of the securities are said to have no correlation; they are completely random.
In real life, perfectly correlated securities are rare, rather you will find securities with some degree of correlation.
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Inverse Correlation
A contrary relationship between two variables such that they ... 
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Positive Correlation
A relationship between two variables in which both variables ... 
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1. A measure of the dispersion of a set of data from its mean. ... 
Covariance
A measure of the degree to which returns on two risky assets ... 
Modern Portfolio Theory  MPT
A theory on how riskaverse investors can construct portfolios ...

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What does a negative correlation coefficient mean?
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How should I interpret a negative correlation?
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How can you calculate correlation using Excel?
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How is correlation used differently in finance and economics?
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