In [[density log|density logging]]. Common rock elements of usual interest in oil exploration, C, O, Si, and Ca all have a Z/A = 1/2, to a very high degree of approximation. For these elements and their molecules, the electron population is closely related to the unit mass or [[density]] of the material. In rocks containing elements in which Z/A differs from the Z/A value selected as a standard, the measured response will be different from the expected value. The amount that the measured [[bulk density]] will differ from the true bulk density of the rock material in question is related to the amount that Z/A differs from the standard. This influence on the interpretation of the density logging response is referred to as the Z/A effect. See also [[density log]] and [[compensated density log]].