Table 2 Contingency table of validated psoriatic arthritis registered in the Danish National Patient Registry (NPR) and Danish Twin Registry (DTR)
Primary source (DTR)
YesNo
Secondary source (NPR)Yes189
No28X
  • Using the capture–recapture method the number of missed psoriatic arthritis cases was X = 9×28/(18+1) = 13. The completeness of the primary source was (18+1)/27 = 70% and of the secondary source it was (18+1)/46 = 41%.