In talking about production/decay processes, I've heard people speaking of decay modes or cross sections being 'phase space' suppressed. For example, a two body final state is more likely to occur than a three body final state, since in the later case the three particles must share the initial energy momentum whereas in the former case only two particles must share.
But, how do I quantify such an argument based purely on the phase space integration that appears in the cross section/decay rate formulas. The two-body phase space integration has dimensions [mass]$^0$ whereas the three-body phase space integration has dimensions [mass]$^2$. How then am I to compare the phase space 'volumes' available to a process if they have different dimensions?