Hypothesis testing has some similarities with one of the simplest methods of proof from mathematical logic: Proof by contradiction. Hypothesis testing can be thought of as being the probabilistic counterpart of proof by contradiction.

Let us refresh our memories first about this method. *How does proof by contradiction work?*

Let’s say we want to prove a statement, call it A. When we use proof by contradiction, we start by assuming the opposite of A (denoted ~A or not A) as being true, and then we try to reach to a contradiction (a statement that is always false). If we reach a contradiction, then this means that our assumption that ~A is true, should be false, and therefore A should be true.

For example, let’s prove by contradiction that there is no smallest positive rational number. Now A is the statement that we want to prove: “there is no smallest rational number greater than 0”. We start by assuming the opposite of A being true, ~A: “there exists a rational number greater than 0 that is the smallest, call it r”. But if r is a rational number > 0, this means that r/2 is also a rational number > 0. Moreover, r/2 is smaller than r. But this contradicts our assumption, ~A, that r is the smallest rational number > 0. This means that ~A is false, and therefore, A is true.

A downside of this method, and of pure mathematical proofs in general, is that it cannot be directly applied in many real-world situations. Let’s say you want to show that a certain type of medical treatment is effective. Can you use a pure mathematical proof to show that? No. There are too many unknowns and too much uncertainty in such situations to be able to show things in a purely mathematical way; that is to prove with no doubt that a certain thing is true.

But you can show that a certain thing **has some probability of being true**. That’s what statistics do, and there is this method in statistics called hypothesis testing that, in my opinion, borrowed the idea of a proof by contradiction and added a probabilistic dimension to it.

The idea of hypothesis testing is similar to that of proof by contradiction, but this time ~A is called the “null hypothesis” and denoted by H₀, and A is called the “alternative hypothesis” and denoted by H₁. Reaching a contradiction (that is, obtaining a result that **cannot be true**) is now reaching a result that **can be true with low probability**. How low this probability should be? An often-used value is 0.05, but it can also be something else, preferably smaller, depending on how confident you want to be of your result; this threshold value is called a **significance level** and is denoted by α. So, if the probability of your observed data given the null hypothesis is ≤ α, then you consider this probability being too low for H₀ to be true; so, you will *reject* H₀ and therefore *accept* H₁. In the case of hypothesis testing, instead of **proving H₁** (that is, with 100% confidence; this is the case for pure mathematical proofs), you are **1-α confident that H₁ is true**.

Note that we can accept only H₁, not H₀. Just because we may fail to reject H₀ sometimes, this does not mean that we accept it. As in the case of proof by contradiction, just because we may not have a good idea of how to reach a contradiction once we assumed ~A to be true, this does not necessarily mean that ~A is true.

Concluding that H₀ is true based on the initial assumption that H₀ is true would be a paradox, therefore we can only reject H₀.

This is my opinion regarding these 2 methods that may seem very distinct at a glance, as they are applied in vastly different situations, but in fact, they are both based on similar logical ideas.

*I hope you found this information interesting and thanks for reading!*

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