Matrix reporting the median χ² for each reference tracer (columns) and each comparison tracer (rows) together with the 16% and 84% variations. The color red refers to the individual χ² values: the darker the color, the worse the value. At the top of the matrix is plotted, for each reference tracer choice, the median with the 16% and 84% variations when summing the tracer walkers following Eq. (14).