( ( His research ) ( ( was ( ( based ( on ( ( ( careful case-studies ) and ) ( clinical observation ) ) ) ) ( ( ( more ( ( than a ) hundred ) ) years ) ( before ( scientists ( could ( explain ( ( the viruses ) themselves ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP$ His) (NN research)) (VP (VBD was) (VP (VBN based) (PP (IN on) (NP (NP (JJ careful) (NNS case-studies)) (CC and) (NP (JJ clinical) (NN observation)))) (SBAR (NP (QP (JJR more) (IN than) (DT a) (CD hundred)) (NNS years)) (IN before) (S (NP (NNS scientists)) (VP (MD could) (VP (VB explain) (S (NP (DT the) (NNS viruses)) (NP (PRP themselves))))))))) (. .)))
His research was based on careful case-studies and clinical observation more than a hundred years before scientists could explain the viruses themselves.
(ROOT (S (NP (NNS Scientists)) (VP (VBP explain) (NP (DT the) (NN world)) (PP (VBN based) (PP (IN on) (NP (NNS observations))))) (. .)))