If a sample has the same proportion of radiocarbon as that of the tree ring, it is safe to conclude that they are of the same age.In practice, tree-ring calibration is not as straightforward due to many factors, the most significant of which is that individual measurements made on the tree rings and the sample have limited precision so a range of possible calendar years is obtained.
In later years, the use of accelerator mass spectrometers and the introduction of high-precision carbon dating have also generated calibration curves.
A high-precision radiocarbon calibration curve published by a laboratory in Belfast, Northern Ireland, used dendrochronology data based on the Irish oak.
Nowadays, the internationally agreed upon calendar calibration curves reach as far back as about 48000 BC (Reimer et.
al., INTCAL13 and Marine13 radiocarbon age calibration curves 0 – 50000 yrs cal BP, Radiocarbon 55(4), 2013).
Results of carbon-14 dating are reported in radiocarbon years, and calibration is needed to convert radiocarbon years into calendar years.