( ( Persons ( who ( have ( negative ( skin ( test results ) ) ) ) ) ) ( ( should ( be ( retested periodically ) ) ) . ) )
(ROOT (S (NP (NP (NNS Persons)) (SBAR (WHNP (WP who)) (S (VP (VBP have) (NP (JJ negative) (NN skin) (NN test) (NNS results)))))) (VP (MD should) (VP (VB be) (VP (VBN retested) (ADVP (RB periodically))))) (. .)))
Persons who have negative skin test results should be retested periodically.
(ROOT (S (NP (PRP We)) (VP (VBP call) (NP (NP (DT an) (JJ incorrect) (NN test) (NN result)) (SBAR (WHNP (WDT that)) (S (VP (MD should) (VP (VB have) (VP (VBN been) (ADJP (JJ positive) (NP (NP (DT a) (-LRB- -LRB-) (NN n) (-RRB- -RRB-)) (ADJP (JJ false) (JJ negative))))))))))) (. .)))