This paper presents a method for obtaining an analytic expression for the density function of observables in multifield models of inflation with sum-separable potentials .
The most striking result is that the density function in general possesses a sharp peak and the location of this peak is only mildly sensitive to the distribution of initial conditions .
A simple argument is given for why this result holds for a more general class of models than just those with sum-separable potentials and why for such models , it is possible to obtain robust predictions for observable quantities .
As an example , the joint density function of the spectral index and running in double quadratic inflation is computed .
For scales leaving the horizon 55 e -folds before the end of inflation , the density function peaks at n _ { s } = 0.967 and \alpha = 0.0006 for the spectral index and running respectively .