There are separate graphs for the southern hemisphere and for calibration of marine data.

graph of radiocarbon dating accuracy-24

The improvements to these curves are based on new data gathered from tree rings, varves, coral, and other studies.

Significant additions to the datasets used for INTCAL13 include non-varved marine foraminifera data, and U-Th dated speleothems.

The INTCAL13 data includes separate curves for the northern and southern hemispheres, as they differ systematically because of the hemisphere effect; there is also a separate marine calibration curve.

Once testing has produced a sample age in radiocarbon years, with an associated error range of plus or minus one standard deviation (usually written as ±σ), the calibration curve can be used to derive a range of calendar ages for the sample.

A third possibility is that the curve is flat for some range of calendar dates; in this case, illustrated by t The method of deriving a calendar year range described above depends solely on the position of the intercepts on the graph.

These are taken to be the boundaries of the 68% confidence range, or one standard deviation.

In this way, an uninterrupted sequence of tree rings can be extended far into the past.

The first such published sequence, based on bristlecone pine tree rings, was created in the 1960s by Wesley Ferguson.

The calibration curve itself has an associated error term, which can be seen on the graph labelled "Calibration error and measurement error".