On the calculation of isochrons

Most isochrons are calculated with well established methods that account for the analytical uncertainties in both x and y. The results of the calculations are slope, intercept, uncertainties for these values, and a measure of line quality (MSWD). The dependence of these values on analytical uncertainties is discussed, and it is shown that the commonly used values for analytical precision are too inaccurate to give reliable results. These problems generally do not cause significant changes in the slope (age), but is is shown that such situations may be possible. Recommendations are given on the presentation and calculation of isochron data.