This matching algorithm was selected because it has been demonstrated to achieve balance across prognostic factors in trials for which randomized allocation to treatment group is not possible. The method involves prospective individual matching of patients that have already been assigned to treatment groups (in this case, Mobile ACE unit and general medical unit). The method provide a methodologically rigorous alternative for achieving balance across treatment groups, with respect to important prognostic factors, in non-randomized trials where randomization is not possible.