Isochron dating bypasses the necessity of knowing the quantity of initial daughter product in the rock by not using that value in the computation.

According to Brent Dalrymple (20-69), "the trick to the isochron diagram is the normalization of both parent and daughter isotope to a third isotope." This third isotope is the non-decay product isotope of the same element as the daughter element.

In the initial state, the graph of daughter isotope to the third isotope versus parent isotope to the third isotope should result in a straight, horizontal line.

Then the computed age based on the accumulation of daughter products will be incorrect (Stasson 1998).

In order to use the valuable information provided by radiometric dating, a new method had to be created that would determine an accurate date and validate the assumptions of radiometric dating. Isotope dating satisfies this requirement, as daughter products do not decay back to the original parent element.

Based on the assumptions of basic radioactive dating, the problem of an unknown initial amount of daughter isotope is eliminated by the definition of the isochron itself. If contamination has occurred within a sample, the ratios from the sample shouldn't fall on a line.

Instead, the points would be in a scatter on the graph.

The first of these assumptions, that all rocks and minerals that formed from the same homogenous mixture have the same age, is not disputed (Overn 2005).

The second assumption of isochrons is that the initial ratios of the daughter isotope to the non-decay product isotope of the same element are uniform throughout the sample.

As time progresses and decay occurs, the number of atoms of the parent isotope decreases, and the number of atoms of the daughter isotope increases accordingly.

The amount of non-decay isotope in the sample does not change.

These assumptions include: 1) the initial amount of the daughter isotope is known, 2) neither parent or daughter product has migrated into, or out of, the closed rock system, and 3) decay has occurred at a constant rate over time.