Figure 75-12 Forest plot summarizing hospital mortality as expressed by risk difference (absolute risk reduction) for individual trials, pooled results of nine trials of patients with severe exacerbations, and two trials of patients with nonsevere exacerbations. Unit of expression equates a reduction of 1.0 as equal to a 100% risk difference or absolute risk reduction. NPPV, noninvasive positive-pressure ventilation. (Adapted from: Keenan SP, Sinuff J, Cook DJ, et al: Which patients with acute exacerbation of COPD benefit from noninvasive PPV? Ann Intern Med 138:861–870, 2003.)