The Direction Method (Jacquez and Oden, 1994) tests for space-time interaction of retrospective, individual level data, and calculates the average direction of advance of a spread of cases. The method is sensitive to a systematic, directional spread of cases. This pattern occurs when an epidemic sweeps through an area. It also arises from geographically localized exposures, with individuals near the source receiving higher doses and showing symptoms before those farther from the source. You cannot infer a directional process from the Direction method; you can, however, determine whether the observed spread of cases tends to be in one direction.
