> Maybe I'm misinterpreting things, but isn't maintaining correct NaCl levels is by definition part of a good working condition? Sounds a bit circular. You're

> Maybe I'm misinterpreting things, but isn't maintaining correct NaCl levels is by definition part of a good working condition? Sounds a bit circular. You're misinterpreting the definition of "good working condition". Good working condition, as I meant it means that it is operating as "designed" (no religious meaning intended), i.e. no malfunctions in any part are identified. It could be perfectly possible that, in good operating condition, the error margin on the NaCl control system allows for NaCl levels that cause hypertension. The hypothesis negates this possibility, and the hypothesis test seems to prove that the error margin on the body's NaCl level control keeps it narrow enough to exclude salt intake as a cause of hypertension (on healthy adults, of course).