The technique is not restricted to tree rings; for example, a stratified tephra sequence in New Zealand, believed to predate human colonization of the islands, has been dated to 1314 AD ± 12 years by wiggle-matching. The wiggles also mean that reading a date from a calibration curve can give more than one answer: this occurs when the curve wiggles up and down enough that the radiocarbon age intercepts the curve in more than one place, which may lead to a radiocarbon result being reported as two separate age ranges, corresponding to the two parts of the curve that the radiocarbon age intercepted. Bayesian statistical techniques can be applied when there are several radiocarbon dates to be calibrated. For example, if a series of radiocarbon dates is taken from different levels in a stratigraphic sequence, Bayesian analysis can be used to evaluate dates which are outliers and can calculate improved probability distributions, based on the prior information that the sequence should be ordered in time. When Bayesian analysis was introduced, its use was limited by the need to use mainframe computers to perform the calculations, but the technique has since been implemented on programs available for personal computers, such as OxCal. Several formats for citing radiocarbon results have been used since the first samples were dated. As of 2019, the standard format required by the journal "Radiocarbon" is as follows. Uncalibrated dates should be reported as ": < year> ±  BP", where:  For example, the uncalibrated date "UtC-2020: 3510 ± 60 BP" indicates that the sample was tested by the Utrecht van der Graaff Laboratorium, where it has a sample number of 2020, and that the uncalibrated age is 3510 years before present, ± 60 years. Related forms are sometimes used: for example, "10 ka BP" means 10,000 radiocarbon years before present (i.e. 8,050 BC), and yr BP might be used to distinguish the uncalibrated date from a date derived from another dating method such as thermoluminescence. Calibrated dates are frequently reported as cal BP, cal BC, or cal AD, again with BP referring to the year 1950 as the zero date. "Radiocarbon" gives two options for reporting calibrated dates. A common format is "cal  ", where:  For example, "cal 1220–1281 AD (1σ)" means a calibrated date for which the true date lies between 1220 AD and 1281 AD, with the confidence level given as 1σ, or one standard deviation.