First, the size of the archaeological sample is important. Larger samples are better, because purification and distillation remove some matter.
Second, great care must be taken in collecting and packing samples to avoid contamination by more recent carbon.
Third, because the decay rate is logarithmic, radiocarbon dating has significant upper and lower limits. It is not very accurate for fairly recent deposits. In recent deposits so little decay has occurred that the error factor (the standard deviation) may be larger than the date obtained. The practical upper limit is about 50,000 years.
Fourth, the ratio of C-14 to C-12 in the atmosphere is not constant. Although it was originally thought that there has always been about the same ratio, radiocarbon samples taken and cross dated using other techniques like dendrochronology have shown that the ratio of C-14 to C-12 has varied significantly during the history of the Earth. To compensate for this variation, dates obtained from radiocarbon laboratories are now corrected using standard calibration tables developed in the past 15-20 years.
