Let's say I am promised that a coin is biased to have probability of heads either $p_1$ and $p_2$. How many times should I toss it to ensure that I know which coin I got, assuming I can be wrong in my conclusion with probability $\varepsilon$?

According to this video, if $p_1 = \frac{1}{d}$ and $p_2 = \frac{2}{d}$ for some $d\geq 2$, one needs $d$ tosses. There is no $\varepsilon$ mentioned there and it is claimed as a straightforward fact. The exact statement is, ""If you want to confidently distinguish between these biases ($\frac{1}{d}$ and $\frac{2}{d}$), you need around $d$ tosses".

Can anyone explain how to arrive at that result and how to generalize it for arbitrary $p_1, p_2$? To be specific:

  1. I am promised that my coin yields heads with probability either $p_1$ or $p_2$.
  2. I have to decide how many times to toss it.
  3. After that many tosses, I can see the results and I should be able to correctly answer which coin I got with error at most $\varepsilon$.
  • 4
    $\begingroup$ The result quoted can't possibly be right. However many tosses there are, you can't obtain certainty. $\endgroup$
    – Mohan
    Commented Dec 4, 2023 at 18:26
  • 1
    $\begingroup$ So $p_1$ and $p_2$ are known? $\endgroup$ Commented Dec 4, 2023 at 18:55
  • $\begingroup$ I don't quite follow the coin analogy, it seems the goal is rather to distinguish between a uniform and non-uniform distribution for all $d$ possible outcomes. In the previous video he gave a formula $E(||p_d-\hat{p_d}||_1) \le \sqrt d / \sqrt n$ alongside the claim that '$n$ modestly bigger than $d$, that is a big constant times $d$' would thus result in 'the expectation of a very small error'. I'm guessing that's what this is building on, and that the '$\approx d$' sample size is more to be interpreted in big-$O$-like notation. The $\sqrt d$ after that is $n\gg \sqrt d$ in the paper as well. $\endgroup$
    – PBulls
    Commented Dec 4, 2023 at 20:28
  • $\begingroup$ @Mohan the quote in the video doesn't say distinguish with certainty. It says, "If you want to confidently distinguish between these biases, you need around $d$ measurements". I didn't understand why exactly, nor how to do it for a general case of $p_1$ and $p_2$ (where both are known). $\endgroup$ Commented Dec 4, 2023 at 22:07
  • $\begingroup$ I don’t know how this can be done rationally without positing a prior distribution for Pr(heads) and a zone of equivalence. Pr(heads) will never be 0.5 exactly, so a Bayesian posterior probability that Pr(heads) is outside of for example [0.49, 0.51] would provide evidence for consequential bias. $\endgroup$ Commented Dec 7, 2023 at 13:18

1 Answer 1


As @PBulls points out in a comment, this quote from the video is probably a very informal summary of some probabilistic bound on $|\hat p - p|$.

One simple example of how to approach this is to use Chebyshev's inequality.
(But I haven't watched the full video; nor the one before it, mentioned in the comments; so I apologize if this doesn't match the video's context exactly.)

You say that you are only considering the two options $p_1$ and $p_2$.
Once you use a sample to estimate $\hat p$, you'll probably want to pick the $p_i$ closest to your $\hat p$.
But the sample could mislead you. Maybe $p_1$ is true, but you get an unlucky sample whose $\hat p$ is closer to $p_2$ just due to sampling variation. This is less likely to happen for larger $n$.

From a Frequentist perspective, you might wonder: Before collecting data, how large should I choose $n$ to be, to minimize the risk of something like this?

Let's say you choose $n$ large enough to ensure that $|\hat p - p| < |p_1 - p_2|/2$ will happen with some high probability $1-\epsilon$. Then you only have at most probability $\epsilon$ of getting a sample in which $\hat p$ is closer to the wrong $p_i$ than to the correct $p_i$.

To do this, you need to write

$$P(|\hat p - p| \geq |p_1-p_2|/2) \leq \epsilon$$

in terms of $n$, and choose $n$ large enough. One way to do this is to rewrite it to line up with Chebyshev's inequality:

$$P(|\hat p - p| \geq k \sqrt{Var(\hat p)}) \leq 1/k^2$$

where $Var(\hat p) = p(1-p)/n$ for the binomial "biased coin flip" setup you describe, and $k$ is up to you.

If you are specifically interested in $p_1=1/d$ and $p_2=2/d$, plug everything into Chebyshev and choose appropriate $k$ to get

$$P(|\hat p - p| \geq 1/(2d)) \leq c (d-1)/n$$

(where the constant $c$ just depends on which $p_i$ you plugged in for $p$). When you set $\epsilon = c (d-1)/n$, you conclude that $n$ basically needs to be a constant multiple of $d$ to ensure a fixed $\epsilon$.

In other words, the video probably doesn't mean that you can literally use $n=d$. It just means that $n$ and $d$ can be on the same order of magnitude. You don't need $n$ to be polynomial or exponential or some other fast-growing function of $d$.

  • 1
    $\begingroup$ To add a few other quotes from the videos: their claim only really works for quite large $d$ (>100 is used, their 'rigged' example in the prior video clearly breaks down if you use $d=2$) and make statements like $\approx d$ being "proportional to $d$, for example $50*d$ or $100*d$" (where I would argue that the latter is actually $d^2$ for $d=100$). Anyway, I very much agree with your closing paragraph. $\endgroup$
    – PBulls
    Commented Dec 7, 2023 at 14:32

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