( ( An ( independent variable ) ) ( ( is ( ( a factor ) ( that ( is ( intentionally ( varied ( by ( the experimenter ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT An) (JJ independent) (NN variable)) (VP (VBZ is) (NP (NP (DT a) (NN factor)) (SBAR (WHNP (WDT that)) (S (VP (VBZ is) (ADJP (RB intentionally) (VBN varied) (PP (IN by) (NP (DT the) (NN experimenter))))))))) (. .)))
An independent variable is a factor that is intentionally varied 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))))))))) (. .)))