The simplest form of isotopic age computation involves substituting three measurements into an equation of four variables, and solving for the fourth.The equation is the one which describes radioactive decay: If one of these assumptions has been violated, the simple computation above yields an incorrect age.Isochron methods avoid the problems which can potentially result from both of the above assumptions.Isochron dating requires a fourth measurement to be taken, which is the amount of a different isotope of the same element as the daughter product of radioactive decay.Now that the mechanics of plotting an isochron have been described, we will discuss the potential problems of the "simple" dating method with respect to isochron methods.The amount of initial wouldn't change over time -- because it would have no parent atoms to produce daughter atoms.

Its composition would be represented as a single point on the isochron plot: Note that the above is somewhat simplified.(Rocks which include several different minerals are excellent for this.) Each group of measurements is plotted as a data point on a graph.The X-axis of the graph is the ratio of in a closed system over time.The better the fit of the data to the line, the lower the uncertainty.For further information on fitting of lines to data (also known as regression analysis), see: Note that the methods used by isotope geologists (as described by York) are much more complicated than those described by Gonick.