( ( ( ( ( ( ( Five or ) ( ten ( factor experiments ) ) ) ( ( are not ) uncommon ) ) , ) and ) ( ( larger experiments ) ( sometimes ( are ( ( done , ) ( although ( ( ( ( ( controlling and ) manipulating ) ( ( so many ) ( experimental variables ) ) ) ( at ( one time ) ) ) ( can ( be difficult ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (NP (CD Five)) (CC or) (NP (CD ten) (NN factor) (NNS experiments))) (VP (VBP are) (RB not) (ADJP (JJ uncommon)))) (, ,) (CC and) (S (NP (JJR larger) (NNS experiments)) (ADVP (RB sometimes)) (VP (VBP are) (VP (VBN done) (, ,) (SBAR (IN although) (S (S (VP (VBG controlling) (CC and) (VBG manipulating) (NP (ADJP (RB so) (JJ many)) (JJ experimental) (NNS variables)) (PP (IN at) (NP (CD one) (NN time))))) (VP (MD can) (VP (VB be) (ADJP (JJ difficult))))))))) (. .)))
Five or ten factor experiments are not uncommon, and larger experiments sometimes are done, although controlling and manipulating so many experimental variables at one time can be difficult.
(ROOT (S (NP (NP (DT The) (NNS values)) (PP (IN of) (NP (JJ independent) (NNS variables)))) (VP (VBP are) (VP (VBN controlled) (PP (IN by) (NP (NP (DT the) (NN experimenter)) (PP (IN over) (NP (NP (DT the) (NN course)) (PP (IN of) (NP (DT an) (NN experiment))))))))) (. .)))