The situation in which <I>X</I> causes changes in <I>Y</I>, which in turn, causes changes in <I>X</I>. It occurs when the time periods used in the analysis (e.g., years rather than weeks) are so long that the direction of causality appears to be simultaneous. One way that has been used to model such a situation is to use <a href="search.pl?p=simultaneous equations">simultaneous equations</a>.<o:p></o:p></P>