For many decades, we have had mechanical and electronic precision instruments for objectively measuring a vast range of physiologic and pathologic functions. They provide an invaluable arsenal for clinicians wanting to estimate health status both in clinical practice and research. In respiratory medicine, where would we be without, for instance, spirometers or blood tests? More recently, measurement principles of physics have been used to develop precision instruments to measure subjective health status. The questionnaires that we use today have been developed and validated so that, if used correctly, they provide accurate assessments not only of patients' clinical status but also of their quality of life.