Sometimes a hypothesis must be made that may be plausible but has not been proven.At other times an additional measurement can eliminate the need for one assumption, although no science can be done without assumptions at some level.For example, the geological formations and dates from surrounding features may suggest that the "true" date can only lie within a certain range.As another example, a date that is obviously wrong would confirm a strong belief in the fundamental unreliability of radioactive dating.Given this confirmation of the confounding factor, the line of observations may be extended until it intersects the curve consisting of the possible values without loss of lead. A variation of this method is also known as lead-lead dating.It does not determine the age of a single sample directly, but the time at which different samples (with differing amount of uranium and lead) were separated from a common pool.If they disagree, it may be because lead has been lost at some point in the history of the sample, for example, if there was an episode of heating above 1000 degrees C.(The mineralogy of zircon makes it highly unlikely that either any uranium is lost from the crystal or that any lead was in the crystal to begin with.) To deal with this possibility, an independent measurement is made from several points in the sample.
This form of dating measures the decay of uranium within igneous zircon over a scale of tens of millions to billions of years.
In either case, there will be a subjective tendency to accept the result, rather than performing additional checks that might reveal unsuspected problems.
Anyone using these methods should be well aware of the conditions for validity, the known confounding factors, and the sources of error.
This method has particular significance because it is the only method that purports to give a value for the age of the Earth (that is, the time at which the Earth and asteroids condensed out of the planetary nebula) rather than only a lower limit on the age of the Earth (that is, the age of the oldest surviving rocks on the Earth).
There are a number of assumptions involved, but if they all hold, when the ratio of Pb for the various samples, the points should lie on a straight line.