Other methods do not require knowing the initial quantities.For example, potassium decays into two different isotopes of argon having different half-lives.C14 is continually being created and decaying, leading to an equilibrium state in the atmosphere.
Uranium 235 decay to lead has a half-life of 713 million years, so it is well suited to dating the universe.
For example, the C14 concentration in the atmosphere depends upon cosmic ray intensity.
To take this into account, a calibration curve is developed using other dating methods to establish the C14 levels over time.
Argon/argon dating works using only the ratio of the concentration of the argon isotopes. For the purposes of this debate, "accurate" means that 95% of the dating errors are within 10% of the measured date, within the time span for which the isotope pair is utilized.
other isotope pairs cover intermediate time periods between the spans for carbon 14 and uranium.
Some radiometric dating methods depend upon knowing the initial amount of the isotope subject to decay.