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 the same as the prevalence in the control group used to establish the negative predictive value, then the two are numerically equal. The following diagram illustrates how the "positive predictive value", negative predictive value, sensitivity, and specificity are related. Note that the positive and negative predictive values can only be estimated using data from a cross-sectional study or other population-based study in which valid prevalence estimates may be obtained. In contrast, the sensitivity and specificity can be estimated from case-control studies. Suppose the fecal occult blood (FOB) screen test is used in 2030 people to look for bowel cancer: The