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.
The output is along the bottom axis; it is a trimodal graph, with peaks at around 710 AD, 740 AD, and 760 AD.
Again, the ranges within the 1σ confidence range are in dark grey, and the ranges within the 2σ confidence range are in light grey.
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.
The solid line is the INTCAL13 calibration curve, and the dotted lines show the standard error range—as with the sample error, this is one standard deviation.
Simply reading off the range of radiocarbon years against the dotted lines, as is shown for sample t Variations in the calibration curve can lead to very different resulting calendar year ranges for samples with different radiocarbon ages.
The alternative is to take the original normal distribution of radiocarbon age ranges and use it to generate a histogram showing the relative probabilities for calendar ages.
This has to be done by numerical methods rather than by a formula because the calibration curve is not describable as a formula.
Programs to perform these calculations include Ox Cal and CALIB.