# COMMUNICATING THE MEANING OF GEOCHRONOLOGIC AGE UNCERTAINTIES TO NON-SPECIALISTS

The ± 5 and ± 3 represent either 95% (2-sigma) or 68% (1-sigma) of the area under a probability curve (which here we will assume to be a Gaussian or normal distribution curve). Ages with large uncertainties have short, flat probability curves and ages with small uncertainties have tall, narrow probability curves. This curve is sometimes misunderstood as the probability that any given age is the true age. It is not. It represents the probability that any given result would be obtained if the experiment were repeated a large number of times.

*Precision* is the fractional uncertainty of all the quantifiable sources of error propagated through all of the age calculation mathematics; for the granite 5/544x100 = 0.92% precision. *Accuracy* is the percentage difference between the measured age and the true age. Because we do not know the true age, accuracy is difficult to evaluate. Precise ages (small uncertainties) may sometimes be inaccurate (wrong). Age uncertainties, therefore, can be thought of more as a measure of the reproducibility of the analytical method than as the limits on the true age. Reliable analytical methods have a long track record of precise, reproducible, and geologically sensible results and are commonly deemed most likely to yield accurate ages.

So are 544 ± 5 Ma and 540 ± 3 Ma the same age or different? The answer is neither it can only be said that if the dating experiments were to be repeated a large number of times, then a certain percentage of the results would fall within any arbitrarily defined age range. While this may not seem a very satisfying answer it resolves the types of dilemmas posed above and leads to a better understanding of the meaning of uncertainties in geochronology.