Electrical resistivity and conductivity results in the important concept of "quasineutrality", which says the density of negative charges is approximately equal to the density of positive charges over large volumes of the plasma ("n" = <Z>"n"), but on the scale of the Debye length there can be charge imbalance. In the special case that "double layers" are formed, the charge separation can extend some tens of Debye lengths. The magnitude of the potentials and electric fields must be determined by means other than simply finding the net charge density. A common example is to assume that the electrons satisfy the Boltzmann relation: Differentiating this
Positive and negative predictive values event that the test makes a negative prediction, and the subject has a positive result under the gold standard. The ideal value of the NPV, with a perfect test, is 1 (100%), and the worst possible value would be zero. The NPV can also be computed from sensitivity, specificity, and prevalence: The complement of the NPV is the (FOR): Although sometimes used synonymously, a "negative predictive value" generally refers to what is established by control groups, while a negative post-test probability rather refers to a probability for an individual. Still, if the individual's pre-test probability of the target condition is