Previous age estimations of period, series, and stage boundaries for the Phanerozoic have usually relied on a subjective approach, in which visual inspection was used to determine each time interval. This subjectivity can be eliminated partially by fitting a piecewise linear regression model of radiometric age on a variable calculated from the biostratigraphic data, enabling the computation of estimates of, and confidence limits for, various boundaries of interest. This method is illustrated by calculating the duration of each Phanerozoic system and thus the quantitative age of all the system boundaries. The method can be extended to investigate any part of the Phanerozoic time scale in more detail, provided sufficient biostratigraphically well-controlled isotopic age data are available, as for example, for the Cenozoic and the Eocene.

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