Stagnant dipoles [10] have only time varying amplitude (dipole magnitude). So the number of unknowns is p(5 + T) where p is the number of dipoles and T is the number of time-samples. In this model, the minimum number of dipoles L (assumed to be smaller than N or T) required to describe the observed responses adequately was determined. The resulting residual between the data matrix M and the corresponding model predictions M^ is expressed as
