The goal of an observational study is to analyze the effect of an exposure. Yet, the apparent effect of the observed exposure could actually be the effect of another process, which could be unknown, unavailable, or unaccounted for in a retrospective analysis. ARDS is a deadly disease with very high mortality and morbidity. However, significant differences in mortality exist among the patients with ARDS when subclassified into mild, moderate, and severe form based on degree of hypoxemia. The authors reported very high in-hospital mortality in this cohort and cited studies to correspond with their mortality data that predate the lung protective ventilation era.2 It is noteworthy that information regarding how many patients received lung protective ventilation is lacking in this study. Contrary to current findings, another study reported predicted mortality of 27%, 32%, and 45% in the mild, moderate, and severe ARDS groups, respectively.3 Therefore, it is difficult to support the hypothesis that mortality in patients with ALI/ARDS and CPE is still high and did not change significantly over the past decade.