METHODS OF VALIDATION
Your choice of an appropriate methodology will depend upon your objectives and the stage of your investigation. Is the purpose of your model to predict whether there be an epidemic, to extrapolate—what might the climate have been like on the primitive Earth, or to elicit causal mechanisms—is development accelerating or decelerating? Which factors are responsible?
Are you still developing the model and selecting variables for inclusion, or are you in the process of estimating model coefficients?
There are three main approaches to validation:
Goodness of fit is no guarantee of predictive success. This is particularly true when an attempt is made to fit a deterministic model to a single realization of a stochastic process. Neyman and Scott  showed that the distribution of galaxies in the observable universe could be accounted for by a two-stage Poisson process. At the initial stage, cluster centers come into existence so that their creation in nonoverlapping regions of time–space takes place independently of one another. At the second stage, the spatial distribution of galaxies about the cluster centers also follows a Poisson distribution.
Alas, our observations ...