Radio active carbon dating
The carbon-14 atoms that cosmic rays create combine with oxygen to form carbon dioxide, which plants absorb naturally and incorporate into plant fibers by photosynthesis.
When this method was first developed, a fairly large amount of carbon was necessary for dating but use of the AMS (accelerator mass spectrometer) today necessitates only a few milligrams for analysis.
This number is called a standard deviation and is a measure of the spread of measurements around the mean (average).
One standard deviation has a 68% probability and two standard deviations have a 95% probability.
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The ratio of normal carbon (carbon-12) to carbon-14 in the air and in all living things at any given time is nearly constant.