This figure is directly based on the proportion of radiocarbon found in the sample.
As explained below, the radiocarbon date tells us when the organism was alive (not when the material was used).
This fact should always be remembered when using radiocarbon dates.
This is very useful as a record of the radiocarbon concentration in the past.
If we have a tree that is 500 years old we can measure the radiocarbon in the 500 rings and see what radiocarbon concentration corresponds to each calendar year.
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.
The wood in these rings once laid down remains unchanged during the life of the tree.
The dating process is always designed to try to extract the carbon from a sample which is most representative of the original organism.
In general it is always better to date a properly identified single entity (such as a cereal grain or an identified bone) rather than a mixture of unidentified organic remains.
In practice this is complicated by two factors: These effects are most clearly seen by looking at a specific example.
This plot shows how the radiocarbon measurement 3000 -30BP would be calibrated.
All animals in the food chain, including carnivores, get their carbon indirectly from plant material, even if it is by eating animals which themselves eat plants.