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There are two main methods used for calculating age ranges from the calibration curve: The first method to be employed was called the `intercept method' because it can be done by drawing intercepts on a graph.
This method will tell you the years in which the radiocarbon concentration of tree rings is within two standard deviations of your measurement (e.g.
See also ORAU's Explanation of Radiocarbon Results.
between 2940BP and 3060BP for the measurement 3000 -30BP).
A slightly different method is now more often used which is called the `probability method'.
The results of calibration are often given as an age range.
In this case, we might say that we could be 95% sure that the sample comes from between 1375 cal BC and 1129 cal BC.
To give an example if a sample is found to have a radiocarbon concentration exactly half of that for material which was modern in 1950 the radiocarbon measurement would be reported as 5568 BP.
For two important reasons, this does not mean that the sample comes from 3619 BC: Many types of tree reliably lay down one tree ring every year.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.For older periods we are able to use other records of with idependent age control to tell us about how radiocarbon changed in the past.The information from measurements on tree rings and other samples of known age (including speleothems, marine corals and samples from sedimentary records with independent dating) are all compiled into calibration curves by the Int Cal group.The wood in these rings once laid down remains unchanged during the life of the tree.