... setting of early studies on prone ventilation, ecological bias consisted of variable prone duration, mixed severity of acute lung injury, variable time-lapse between lung injury onset and inclusion, ... patients (ARDS only, excluding acute lung injury (ALI) non -ARDS patients), and control for the most relevant confounders - that is, proning duration (usually >17 hours/day) and use of lung- protective ... towards an interaction between longer proning duration and reduction in mortality The initial studies by Guerin and colleagues [2] and Gattinoni and colleagues [1] had the greatest impact on...