Avoidable statistical pitfalls in analyzing length of stay in intensive care units or hospitals

In a study of ventilated patients in a recent issue of Critical Care, Shahin and colleagues [1] compared the length of stay in ICU between patients who acquired a suspected ventilator-associated respiratory infection (sVARI) and patients who remained free of this infection. There are two basic facts that require special attention in the statistical analysis. First, ventilation is a timedependent study entry since some patients receive ventilation a few days after the time of admission to the ICU. In keeping with the study design, these patients cannot be discharged between admission and first ventilation; ignoring this fact leads to length bias [2]. Second, sVARI is a time-dependent exposure since sVARI might occur a few days after the start of ventilation; treating sVARI incorrectly as time-fixed (that is, assumed to be known