I was reading an article about the polar motion of the Earth: https://www.researchgate.net/publication/241368199_The_Earth's_variable_Chandler_wobble. A regression is performed between observed and modeled complex time series of data - the type of regression is not specified. The regression coefficient is then calculated for different values of the estimated parameter (quality factor); its amplitude and phase are shown in Figure 2 (I don't know if I am allowed to post it directly here).
My question is why can the amplitude of the regression coefficient be as high as 2.3? I thought that the absolute value of the regression coefficient, for real series, had to be in the range 0-1. Therefore I was expecting a maximum amplitude of $\sqrt{2}$ for a complex series.
Is there a more general definition of regression coefficient that I'm missing?
