Con los criterios indirectos de valoración: 2 + 2 ó 2 x 2 no es cuatro...

Referencias: 

1. Fleming TR, DeMets DL. Surrogate end points in clinical trials: are we being misled? Annals of Internal Medicine. 1996; 125(7):605-13.
2. Del Mar C. ACP Journal Club. Review: primary care-based general health checks improve surrogate but not clinical outcomes. Annals of Internal Medicine. 2014; 160(12):JC5.