<P> The calculation uses Libby's half - life of 5,568 years, not the more accurate modern value of 5,730 years . Libby's value for the half - life is used to maintain consistency with early radiocarbon testing results; calibration curves include a correction for this, so the accuracy of final reported calendar ages is assured . </P> <P> The reliability of the results can be improved by lengthening the testing time . For example, if counting beta decays for 250 minutes is enough to give an error of ± 80 years, with 68% confidence, then doubling the counting time to 500 minutes will allow a sample with only half as much C to be measured with the same error term of 80 years . </P> <P> Radiocarbon dating is generally limited to dating samples no more than 50,000 years old, as samples older than that have insufficient C to be measurable . Older dates have been obtained by using special sample preparation techniques, large samples, and very long measurement times . These techniques can allow measurement of dates up to 60,000 and in some cases up to 75,000 years before the present . </P> <P> Radiocarbon dates are generally presented with a range of one standard deviation (usually represented by the Greek letter sigma as 1σ) on either side of the mean . However, a date range of 1σ represents only 68% confidence level, so the true age of the object being measured may lie outside the range of dates quoted . This was demonstrated in 1970 by an experiment run by the British Museum radiocarbon laboratory, in which weekly measurements were taken on the same sample for six months . The results varied widely (though consistently with a normal distribution of errors in the measurements), and included multiple date ranges (of 1σ confidence) that did not overlap with each other . The measurements included one with a range from about 4250 to about 4390 years ago, and another with a range from about 4520 to about 4690 . </P>

State one limitation on the use of carbon 14 for dating ancient artifacts