Figure 2: Constraints of DESY3 data with and without external data on the Neff-meff model. Contours of the same color represent the same data where each contour represents the likelihood that the true value is within that enclosed region. The smaller contours tell us that the true value is within this region with an 68% certainty and the larger contours with a 95% certainty. The blank region that is slashed-out on the left represents the unphysical region of this model, described in the previous section. We find DESY3 data alone does not constrain this model much, but with the inclusion of data from other probes we obtain the tightest constraints on the mass of a sterile neutrino to-date!