All of these secondary factors (preparations, etc.) can themselves be regarded as "random" or at least unknowable in their extent and their effects. Much like lifetimes of individuals -- having some clear predictability on an individual basis, but in aggregate, well-modeled as random variables.
To be a bit more abstract, just because something is "random" does not mean it cannot be characterized with a distribution, expected life, etc.
The philosophical/mathematical effort at quantifying randomness has been called "The Taming of Chance".
The effectiveness of preparation is not random. Charting the effectiveness of such efforts, were it possible to do so, might yield a random distribution; this does not, however, imply that "all of these secondary factors" can or should be "regarded as random" for anything more than a high-level treatment that becomes worthless in the (individual) context where it actually matters.
I tried to have a very light touch with the data. Most of what I do is non-parametric -- so in this sense, yes as an individual this analysis is only good if you believe there's some similarity between you and everyone that's ever been on the list (so a finite sample).
To be a bit more abstract, just because something is "random" does not mean it cannot be characterized with a distribution, expected life, etc.
The philosophical/mathematical effort at quantifying randomness has been called "The Taming of Chance".