by DragonGod1 min read21st Jun 2017No comments


Personal Blog


This post contains Latex. Please install Tex the World for Chromium or other similar Tex typesetting extensions to view this post properly.

Priors are Useless.

Priors are irrelevant. Given two different prior probabilities [;Pr_{i_1};], and [;Pr_{i_2};] for some hypothesis [;H_i;].
Let their respective posterior probabilities be [;Pr_{i_{z1}};] and [;Pr_{i_{z2};].
After sufficient number of experiments, the posterior probability [;Pr_{i_{z1}} \approx [;Pr_{i_{z2};].
Or More formally:
[;\lim_{n \to \infty} \frac{ Pr_{i_{z1}}}{Pr_{i_{z2}}} = 1 ;].
Where [;n;] is the number of experiments.
Therefore, priors are useless.
The above is true, because as we carry out subsequent experiments, the posterior probability [;Pr_{i_{z1_j}};] gets closer and closer to the true probability of the hypothesis [;Pr_i;]. The same holds true for [;Pr_{i_{z2_j}};]. As such, if you have access to a sufficient number of experiments the initial prior hypothesis you assigned the experiment is irrelevant.
To demonstrate.
This is the graph of the above table:\_C2aInqzqblnA.png
In the example above, the true probability of Hypothesis [;H_i;] [;(P_i);] is [;0.5;] and as we see, after sufficient number of trials, the different [;Pr_{i_{z1_j}};]s get closer to [;0.5;].
To generalize from my above argument:

If you have enough information, your initial beliefs are irrelevant—you will arrive at the same final beliefs.
Because I can’t resist, a corollary to Aumann’s agreement theorem.
Given sufficient information, two rationalists will always arrive at the same final beliefs irrespective of their initial beliefs.

The above can be generalized to what I call the “Universal Agreement Theorem”:

Given sufficient evidence, all rationalists will arrive at the same set of beliefs regarding a phenomenon irrespective of their initial set of beliefs regarding said phenomenon.


Exercise For the Reader

Prove [;\lim_{n \to \infty} \frac{ Pr_{i_{z1}}}{Pr_{i_{z2}}} = 1 ;].

Personal Blog


New Comment