With uranium-lead dating, for example, the process assumes the original proportion of uranium in the sample.
One assumption that can be made is that all the lead in the sample was once uranium, but if there was lead there to start with, this assumption is not valid, and any date based on that assumption will be incorrect (too old).
In the case of carbon dating, it is not the initial quantity that is important, but the initial ratio of C, but the same principle otherwise applies.
Recognizing this problem, scientists try to focus on rocks that do not contain the decay product originally.
For example, in uranium-lead dating, they use rocks containing zircon (Zr Si O Zircon and baddeleyite incorporate uranium atoms into their crystalline structure as substitutes for zirconium, but strongly reject lead.Zincon has a very high closure temperature, is very chemically inert, and is resistant to mechanical weathering.Because radiometric dating fails to satisfy standards of testability and falsifiability, claims based on radiometric dating may fail to qualify under the Daubert standard for court-admissible scientific evidence.It is more accurate for shorter time periods (e.g., hundreds of years) during which control variables are less likely to change.
Radiometric dating is a method of determining the age of an artifact by assuming that on average decay rates have been constant (see below for the flaws in that assumption) and measuring the amount of radioactive decay that has occurred.
Radiometric dating is mostly used to determine the age of rocks, though a particular form of radiometric dating—called Radiocarbon dating—can date wood, cloth, skeletons, and other organic material.