In daily practice the expiratory flow limitation is measured by body plethysmography. An open loop in body plethysmography shows expiratory flow limitation in COPD patients. But body plethysmography requires that the patient quietly sits in a closed box and is not able to follow changes breath by breath. The goal of this study is to validate the Forced oscillation technique (FOT) for measuring expiratory flow limitation in COPD patients.30 COPD patients with an open loop in body plethysmography are compared to 30 COPD patients with a closed loop. The methods used in this study are the maximal overlap discrete short time Fourier transform (MODFT) combined with a total least squares analysis. It is hypothesized that the obtained FOT parameters are able to distinguish the patients with expiratory flow limitation in body plethysmography from the patients without expiratory flow limitation in body plethysmography.Secondary study questions are: 1) to what extend are the FOT parameters distorted by breathing artifacts and measurement noise. 2)What changes can be detected by FOT in expiratory flow limitation after administration of salbutamol.