Editing Pearson correlation coefficient (section)

===Existence===
The population Pearson correlation coefficient is defined in terms of [[moment (mathematics)|moments]], and therefore exists for any bivariate [[probability distribution]] for which the [[statistical population|population]] [[covariance]] is defined and the [[marginal distribution|marginal]] [[population variance]]s are defined and are non-zero. Some probability distributions, such as the [[Cauchy distribution]], have undefined variance and hence ρ is not defined if ''X'' or ''Y'' follows such a distribution. In some practical applications, such as those involving data suspected to follow a [[heavy-tailed distribution]], this is an important consideration.  However, the existence of the correlation coefficient is usually not a concern; for instance, if the range of the distribution is bounded, ρ is always defined.