In many cases, there are independent cues (such as geologic setting or the chemistry of the specimen) which can suggest that such assumptions are entirely reasonable.
My latest guide, "The Newbie's Guide To Everlasting Love", is now available on Kindle. Contact me using this form and get 50% off your first session with me.
Contact me here for coaching opportunities and media coverage.
The simplest form of isotopic age computation involves substituting three measurements into an equation of four variables, and solving for the fourth.
The data points would be expected to start out on a line if certain initial conditions were met.
Consider some molten rock in which isotopes and elements are distributed in a reasonably homogeneous manner.