A luminescence age from a sample is typically calculated from a distribution of ages measured from multiple individual grains or multi-grain aliquots from that sample.
The overdispersion of the De distribution (commonly abbreviated “OD” or “σ”) represents the “spread” in the distribution that remains after all measurement errors specific to each grain/aliquot (also known as the “within-aliquot variation”) have been taken into account (Galbraith and Roberts, 2012). Below, we show an example of a sample with low OD, and a sample with high OD.
When σ is non-zero or high, we may infer that:
i) some grains have been insufficiently bleached prior to deposition,
ii) there has been mixing between sedimentary layers of different ages, and/or
iii) there are grain-to-grain variations in environmental dose rate not taken into account in our dose rate estimate.