In archaeology the term calibration is used to describe the process of increasing the accuracy of a dating technique by calculating and correcting a known error. In radiocarbon (C14) dating, for example, it was assumed that the natural occurrence of C14 has been constant over time. However this has been shown to be untrue and consequently radiocarbon dates are often incorrect. The fluctuations in radiocarbon have been mapped and compared with dendrochronological measurements in order to correct dates obtained using this technique. Using this technique it is possible to give a calendar date and a range of years either side to which the item may also date but which is proportionally less likely the further away from the middle date these years are.