Huge progress has been made over the past decades in introducing patient reported outcomes (PROs) in medicine. Such measures capture the patient’s own perspective of disease and health. PRO measurement includes multiple domains (i.e. physical, mental and social life aspects), do not only focus on disorders, and include well-being. PRO instruments can be generic or disease-specific, investigator-driven or individualized. Individualized instrument enhance the relevance of assessment by allowing the patient to determine the content, may have a higher clinical relevance, but reduce the comparability of results. Investigator-driven instruments are well suited for clinical trials but may neglect specific individual needs. Generic PRO measures are independent of a particular health condition allowing across-disease comparisons. However, they may lack sensitivity for a particular condition and may be less relevant to patients than disease-specific measures. Further, using more than one outcome measure creates problems for traditional methodological and statistical approaches. Therefore a single measure to determine the effect size and power of a study is over simplistic. Choosing a measure that is likely to change over time may lead to underpowered results for other outcome measures used. However, by using the measure that is least likely to change, the trial may become impractical
