(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 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.
Each such age would match the result given by the isochron.
Gain or loss of In order to make the figures easy to read (and quick to draw), the examples in this paper include few data points.
Consider some molten rock in which isotopes and elements are distributed in a reasonably homogeneous manner.
The "generic" method described by Gonick is easier to understand, but it does not handle such necessities as: (1) varying levels of uncertainty in the X- versus Y-measurements of the data; (2) computing an uncertainty in slope and Y-intercept from the data; and (3) testing whether the "fit" of the data to the line is good enough to imply that the isochron yields a valid age.
It depends on the accuracy of the measurements and the fit of the data to the line in each individual case.) For example, with Rb/Sr isochron dating, any age less than a few tens of millions of years is usually indistinguishable from zero.
That encompasses the entire young-Earth timescale thousands of times over." in the decay equation.
Since the data points have the same Y-value and a range of X-values, they initially fall on a horizontal line: half-lives will include zero within its range of uncertainty.
(The range of uncertainty varies, and may be as much as an order of magnitude different from the approximate value above.