Radiometric dating is based upon the known half life of certain radioactive isotopes that occur in nature.  Half life is expressed as a constant number of years required for half of the mother element to decay into the daughter element.  The amount of each element present in a rock could give a good indication as to the age of the rock.  Depending on the supposed age of the rock in question, certain elements must be chosen which have a half life long enough to yield measurable ages.  What is not always explained however is that it is necessary to base this dating technique upon three assumptions:

1. The ratio of parent to daughter element at the time the rock is formed.
2. No changes occur over time affecting the relative amounts of those elements except for their natural decay process.
3. The natural rate of decay from mother to daughter element has always been the same as is observed today.

If any one of these assumptions are incorrect the supposed age of the rock may be off by several orders of magnitude.  It is also not unusual to test different rocks from the same strata and receive widely divergent results.  In addition to this, there is evidence to suggest that the rate of decay may have been much faster in the past, prior to the last 90 years that records have been kept.