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Consider tests of randomness of bit sequences of fixed size $n$ bits as in cryptography (e.g. NIST Special Publication 800-22 page 1-4; notice this uses captital-$P$ for p-values). Define such test as any deterministic function $T$ that accepts a vector $V$ of $n$ bits, and outputs a p-value $p$ in $]0\dots1]$ (with null hypothesis that $V$ consists of random independent unbiased bits), that is with the defining property $$\forall\alpha\in[0\dots1],\;\;\Pr\big(T(V)\le\alpha\big)\,\le\,\alpha$$ where the probability is computed with $V$ a vector of random independent unbiased bits (or equivalently, is computed as the proportion of $V$ such that $T(V)\le\alpha$ among the $2^n$ vectors $V$).

Example tests matching this definition are

  • True, which always output $p=1$.
  • Non-zero, which outputs $p=1/2^n$ if all bits in $V$ are zero, and outputs $p=1$ otherwise.
  • Non-stuck, which outputs $p=1/2^{n-1}$ if all bits in $V$ are identical, and outputs $p=1$ otherwise.
  • Balanced, which computes the number $s$ of bits set in $V$ (that is the Hamming weight of $V$), and outputs as $p$ the odds that for random $V'$, $|2s'-n|$ is at least $|2s-n|$.
    • For $n\le3$, Balanced is the same as Non-stuck.
    • For $n=4$, $p=\begin{cases} {1/8}&\text{ if } s\in\{0,4\}&\text{e.g. }V=(0,0,0,0)\\ {5/8}&\text{ if } s\in\{1,3\}&\text{e.g. }V=(1,0,1,1)\\ 1&\text{ otherwise}&\text{e.g. }V=(1,0,0,1)\end{cases}$
    • For $n=5$, $p=\begin{cases} {1/16}&\text{ if } s\in\{0,5\}&\text{e.g. }V=(1,1,1,1,1)\\ {3/8}&\text{ if } s\in\{1,4\}&\text{e.g. }V=(0,0,1,0,0)\\ 1&\text{ otherwise}&\text{e.g. }V=(0,1,1,0,1)\end{cases}$

There's a natural partial order relation among tests: $T$ implies $T'$ when $\forall V, T(V)\le T'(V)$. Any test implies True. Balanced implies Non-stuck, but does not imply Non-zero. Some tests, including Balanced and Non-zero, are optimal in the sense that no other test implies them.

Section 2 of the above reference describes 15 tests for large $n$ (thousands bits), that are intended to catch some defects relevant to actual random number generators, and be near-optimal (in the above sense). For example, section 2.1 is an approximation of Balanced for large $n$ using the complementary error function, designated The Frequency (Monobit) Test.

Q1: Assume that all the $n$ bits in a vector $V$ tested are random independent bits having same odds $q={1\over2}+\epsilon$ to be set, with $\epsilon$ unknown (besides being smallish), corresponding to Shannon entropy per bit $$E=-q\log_2(q)-(1-q)\log_2(1-q)=1-{2\over\log2}\epsilon^2+\mathcal O(\epsilon^4)$$

The Balanced test for some (large) number $n$ of such bits is applied once, and outputs a small p-value (say $p\le0.001$). That allows us to reject the null hypothesis $H_0$ that $E=1$ (equivalently, $\epsilon=0$) with high confidence (corresponding to the p-value $p$).

What is a tight function $E(p,n)$ such that we can reject the hypothesis $H_1$ that $E\ge E(p,n)$ with some good confidence (corresponding to some known p-value higher than $p$, perhaps $2p$, or $\sqrt p$, or something on that tune)? By "tight function" I mean that the lowest $E(p,n)$ we manage to prove for some confidence, the better.

Q2: Things are as in Q1, except that the test is unspecified beyond the defining property of p-values. Can we reject the hypothesis $H_1$ that $E\ge E(p,n)$ with good confidence, for whatever $E(p,n)$ and at least the confidence level that was established in Q1? If that conjecture was false, what's a counterexample, or/and is that reparable?

Q3: Things are as in Q2 (or in Q1 if the property thought in Q2 does not apply), except that the bits in the input $V$ might be dependent, but still with Shannon entropy per bit $E$; that is, the distribution of the inputs $V$ is such that $$nE\;=\;-\sum_{V\text{ with }\Pr(V)\ne0}\Pr(V)\log_2(\Pr(V))$$ Can we reject the hypothesis $H_1$ that $E\ge E(p,n)$ with good confidence, for whatever $E(p,n)$ and at least the confidence level that was established in Q1? If that conjecture was false, what's a counterexample, or/and is that reparable?

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  • $\begingroup$ The question is an hopefully clearer variant of this question on cse, which has an answer for Q1 that I'm having some difficulty to follow. $\endgroup$
    – fgrieu
    Mar 7 '17 at 17:29
  • $\begingroup$ The probability that all possible events happen is 1. Can the Balanced Test be stated in this way? Could you give an example of a bit vector for a given $n$? Describe what is meant by "the number s of bits set in V". Does "per bit" imply a value for each binary digit? In statistics, capital $P$ means probability and small $p$ indicates the p-value. $H_0$ is the null hypothesis, $H_1$ is the alternative hypothesis, etc. I suggest this notation. Q1: What is meant by "all bits"? All bits in a byte, word, sample, or the universe? $\endgroup$
    – noumenal
    Mar 14 '17 at 22:38
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    $\begingroup$ @noumenal: Following your advise I now use $p$ for p-value, even though my reference uses $P$; $E$ for entropy, leaving $H_0$ for the null hypothesis and $H_1$ for an hypothesis made with knowledge of the p-value. I tried to make the definition of Balanced clearer, including giving alternate definition of "the number $s$ of bits set in $V$", and examples of $V$. I hopefully clarified "all bits" in Q1. $\endgroup$
    – fgrieu
    Mar 14 '17 at 23:31
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If I understand your problem correctly, the choice would be multinomial logistic regression, also known as the "conditional maximum entropy model". Concerning your second question I guess that two different concerns can be emphasized: the accuracy of the confidence interval or the statistical power of the inference. I would especially be concerned with correcting for multiple testing, for example using the Holm-Bonferroni correction of the alpha values or something more contemporary.

For the final question, a method that takes prior probabilities into account would be a Bayesian method. Although I don't have a concrete suggestion here, the problem reminds me of random forests and naive Bayes classifiers. However, these models not only involve dependence, but training, so I am not sure if those topics would be relevant to the problem at hand.

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  • $\begingroup$ That might be a path to solutions, but it flies far over my head, and is hardly an answer. Thanks for your effort, and having helped improve the question. $\endgroup$
    – fgrieu
    Mar 17 '17 at 12:11
  • $\begingroup$ Perhaps you are making this more complicated than it is. For a certain data type (e.g. multiple categories of unordered data) and a certain problem (e.g Is there a significant difference?) there is a test, as the one I mentioned above. Perhaps if you could state your problem in plain English, you would demonstrate that you have understood the problem and would make it easier for me and others to help you. So far I have only understood that you wish to optimize some cost by defining a function that minimizes some criterion. How does this relate to the real world? $\endgroup$
    – noumenal
    Mar 18 '17 at 13:48

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