An <a href="search.pl?p=explanatory variable">explanatory variable</a> that should be part of a model but has been excluded. Its exclusion can lead to biased and inefficient estimates of the remaining <a href="search.pl?p=parameters">parameters</a> in the model. Omitting it causes no problem in the estimation of the included variables if it is constant for the <a href="search.pl?p=calibration data">calibration data</a>, or if its variations are uncorrelated with the included variables. Its exclusion can lead to inaccurate forecasts if it changes over the forecast horizon.<o:p></o:p></P>