Lava flows (and other igneous formations) make great yard sticks for radiometric dating because heated rocks release the daughter isotopes whose ratio is used for dating. Hence, lava can be dated most accurately and rock formations above ancient lava flows would have to be younger, while those below would be older. Also, this is another reason that multiple tests are conducted on the same samples using different methods. Even if there was a skew in one parent-daughter ratio, that exact same skew would not be present in another parent-daughter ratio. If several methods converge on a specific date, odds are that's how old the rock is.