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.
In Accelerator Mass Spectrometry (AMS), for example, the number of radiocarbon atoms in a stream of atoms coming from the sample is counted.
Thus there are statistical counting uncertainties proportional to the square root of the number of atoms counted.
Radiocarbon dates can be obtained from many types of organic material including charcoal, shell, wood, bone and hair.
The amount of carbon dioxide in the living organism is equal to that in the atmosphere.
For example, it is well known that carbon dating cannot be used on many types of marine life due to reservoirs of "old" carbon held in sedimentary rocks.That is, samples with dates known from historical records can be used to check the accuracy of the method.Despite this, however, caution is still necessary in accepting dates derived from carbon dating.Archeologists use several methods to establish absolute chronology including radiocarbon dating, obsidian hydration, thermoluminescence, dendrochronology, historical records, mean ceramic dating, and pipe stem dating.Each of these methods is explained in this section.