The natural statistical distribution of a series of measurements around an arithmetic mean value; a measure of the scatter (variability, dispersion, spread) about the mean in a distribution. In archaeology, it is used in association with chronometric dating techniques like radiocarbon dating, where each measurement is a calculation of date for the sample, and the final date given, e.g. 2,400 ? 200, is a statistical description of a 'real' date. The standard deviation (?) as quoted means that there is a 66% chance of the real date lying within that range (for the above example, between 2,600-2,200). For greater probability, the date must be taken to two standard deviations (there is a 95% certainty that the date lies between 2,800-2,000) or three standard deviations (99% certainty). A single date with a relatively large error is generally of less use than a series of dates from the same context, which may show a clustering around a central date.