PT - JOURNAL ARTICLE AU - Perren, Andreas AU - Merlani, Paolo TI - The Difficulty of Measuring the Measurable AID - 10.4187/respcare.03380 DP - 2014 Jun 03 TA - Respiratory Care PG - respcare.03380 4099 - http://rc.rcjournal.com/content/early/2014/06/03/respcare.03380.short 4100 - http://rc.rcjournal.com/content/early/2014/06/03/respcare.03380.full