( ( The ( independent variable ) ) ( ( is ( that ( which ( is ( ( ( controlled or ) manipulated ) ( by ( the experimenter ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (JJ independent) (NN variable)) (VP (VBZ is) (NP (NP (DT that)) (SBAR (WHNP (WDT which)) (S (VP (VBZ is) (VP (VBN controlled) (CC or) (VBN manipulated) (PP (IN by) (NP (DT the) (NN experimenter))))))))) (. .)))
The independent variable is that which is controlled or manipulated by the experimenter.
(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))))))))) (. .)))