Dating a rock
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.
Note that the mere existence of these assumptions do not render the simpler dating methods entirely useless.
especially in absence of cross-checks by different methods, or if presented without sufficient information to judge the context in which it was obtained.
Isochron methods avoid the problems which can potentially result from both of the above assumptions.
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(The range of uncertainty varies, and may be as much as an order of magnitude different from the approximate value above.
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.
A routine statistical operation on the set of data yields both a slope of the best-fit line (an age) and a variance in the slope (an uncertainty in the age).
The better the fit of the data to the line, the lower the uncertainty.