A measure of how closely associated two variables are and whether they tend to take similar or dissimilar values, ranging from a value of 1 indicating that the variables take similar values (‘are positively correlated’) to –1 indicating that the variables take dissimilar variables (‘negative’ or ‘inverse’ correlation). A value of 1 or –1 indicates that knowing the value of one of the variables would allow you to perfectly predict the value of the other. A value of 0 indicates that knowing one of the variables provides no information about the value of the other. See also: correlation, causality.