the illness was caused by an adverse reaction to a vaccination . The credibility of a witness can be impeached by a showing of interest , bias , inconsistent statements , or , to a certain extent , bad character . See Caluza v . Brown , 7 Vet . App . 498 , 511 ( 1995 ) , aff ' d per curiam , 78 F . 3 d 604 ( Fed . Cir . 1996 ) . Here , the Veteran ' s assertions are both inconsistent and lacking in probative value due to the Veteran ' s lack of medical training or expertise to allow her to form a medical judgment regarding the cause of her son ' s illness . The Board finds no