Radiocarbon dating measurements produce ages in "radiocarbon years", which must be converted to calendar ages by a process called calibration. Calibration is needed because the atmospheric / ratio, which is a key element in calculating radiocarbon ages, has not been constant historically. Although Willard Libby, the inventor of radiocarbon dating, had pointed out as early as 1955 the possibility that the / ratio might have varied over time, it was not until discrepancies began to accumulate between measured ages and known historical dates for artefacts that it became clear that a correction would need to be applied to radiocarbon ages to obtain calendar dates. Radiocarbon years ago may be abbreviated ya (years ago). A general term used reflecting evidence from any method is Before Present (BP).