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Once calibrated a radiocarbon date should be expressed in terms of cal BC, cal AD or cal BP.
The cal prefix indicates that the dates are the result of radiocarbon calibration using tree ring data.
See also ORAU's Explanation of Radiocarbon Results.
Radiocarbon dates should always be reported either as `percent modern' or years `before present' (BP).
These values should correspond exactly to normal historical years BC and AD.
In practice this is complicated by two factors: These effects are most clearly seen by looking at a specific example.
The pair of blue curves show the radiocarbon measurements on the tree rings (plus and minus one standard deviation) and the red curve on the left indicates the radiocarbon concentration in the sample.
The grey histogram shows possible ages for the sample (the higher the histogram the more likely that age is).
between 2940BP and 3060BP for the measurement 3000 -30BP).
A slightly different method is now more often used which is called the `probability method'.