Complex sentence: Therefore it is helpful to eliminate some of the unknowns; for example, the "n" and "k" spectra of one or more of the films may be held fixed (not allowed to vary) during the regression.
Split into simpler sentences: Therefore it is helpful to eliminate some of the unknowns. For example, the "n" and "k" spectra of one or more of the films may be held fixed (not allowed to vary) during the regression.