Another important parameter needed for quantification of is the tissue reduced scattering coefficient, ’, i.e., the reciprocal of the photon random walk step length. Although is influenced by both tissue absorption and scattering coefficients, and ’, respectively, uncertainties in lead to relatively small errors in (), whereas uncertainties in ’ can translate to substantial errors in .26 Thus, when quantifying an absolute , it is desirable to combine DCS with a method to determine absolute and ’ for the tissue of interest, e.g., frequency- or time-domain NIRS. We note, however, that in neonates, Jain et al. observed a strong correlation between and CBF in the sagittal sinus despite assuming a fixed ’ across the entire cohort [see Fig. 2(b)].12 This agreement, at least in part, may have been a result of the homogeneity of the patient cohort, i.e., all were week neonates with congenital heart defects. Clearly, in future studies it will be better to measure ’ in all subjects whenever possible. (Note: the effects of uncertainties in and ’ on are diminished when monitoring relative changes in over time, since these tissue parameters change little except in cases of extreme interval blood loss/gain, edema, etc).