Following Cliff and Ord (1981), a variable exhibits spatial autocorrelation
or spatial association if the presence of some quantity in a county
(sampling unit) makes its presence in neighboring counties (sampling
units) more or less likely; i.e., neighboring or nearby areas are more
alike large or small values (positive spatial autocorrelation). Negative
spatial autocorrelation describes patterns in which neighboring areas
are unlike. Random patterns exhibit no spatial autocorrelation.