This is frequently because the selected technique is used outside of its appropriate range, for example on very recent lavas. The Institute for Creation Research's RATE project aimed to show scientifically that methods of radiometric dating produced wildly inconsistent and incorrect values.
Ultimately these "creation scientists" were forced to admit that even for methods they accepted as sound, the age of the Earth would be vastly greater than the 6,000 they set out to prove.
To date older fossils, other methods are used, such as potassium-argon or argon-argon dating.
Radiometric dating — through processes similar to those outlined in the example problem above — frequently reveals that rocks, fossils, etc.
We can measure directly, for example by using a radiation detector, and obtain a good estimate of by analyzing the chemical composition of the sample.
The half-life , specific to each nuclide, can be accurately measured on a pure sample, and is known to be independent of the chemical composition of the sample, temperature and pressure.
One way Young Earth Creationists and other denialists try to discredit radiometric dating is to cite examples radiometric dating techniques providing inaccurate results. Helens, creationists attempted discredit the discipline through dishonest practices.Although the time at which any individual atom will decay cannot be forecast, the time in which any given percentage of a sample will decay can be calculated to varying degrees of accuracy.The time that it takes for half of a sample to decay is known as the half life of the isotope.Some isotopes have half lives longer than the present age of the universe, but they are still subject to the same laws of quantum physics and will eventually decay, even if doing so at a time when all remaining atoms in the universe are separated by astronomical distances.Various elements are used for dating different time periods; ones with relatively short half-lives like carbon-14 (or C) are useful for dating once-living objects (since they include atmospheric carbon from when they were alive) from about ten to fifty thousand years old. Longer-lived isotopes provide dating information for much older times.