[American Thoracic Society American Thoracic Society 2011 International Conference, May 13-18, 2011 • Denver Colorado - ()] B94. CLINICAL RESEARCH IN CRITICAL CARE - Meta-Analysis, Meta-Regression, And Patient- And Trial- Level Data Subgroup Analysis: An Example Of Ecological Bias In A Meta-Analysis Of Prone Ventilation In Patients With Severe Hypoxemic Respiratory Failure Due To ALI/ARDS