The validation of the USD-4D is performed in multiple studies
1)  What do patients need to use the USD-4D in daily care
Qualitative data from individual interviews with 13 patients
2)  What do healthcare professionals need to work with the USD4D in daily care
Quantitative data from a 49-item survey  completed by 122 healthcare professionals
3)  Content validity of the USD-4D from the patient perspective
Qualitative data from interviews with 12 patients
4)  Content validity of the USD-4D from the health care professional perspective
Quantitative data from a survey completed by 601 healthcare professionals
5)  Construct validity of the USD-4D using hypotheses testing 
Quantitative data were used from two cohorts of patients with life limiting illness.