witnesses are also competent to opine on some questions of etiology , and the Board must determine on a case - by - case basis whether a particular condition is the type of condition that is within the competence of a lay person . See Davidson v . Shinseki , 581 F . 3 d 1313 , 1316 ( Fed . Cir . 2009 ) ; Kahana v . Shinseki , 24 Vet . App . 428 , 433 , n . 4 ( 2011 ) . The question of whether the Veteran ' s current psychiatric disorder is related to her in - service experiences is testimony as to an internal psychiatric process , unlike the immediately observable cause - and - effect relationship of the type