Okay, you've set up your null hypothesis ( $H_0$ ) and alternative hypothesis ( $H_1$ ). You understand that when making a decision based on your sample data, you might make a Type I error (rejecting a true $H_0$ ) or a Type II error (failing to reject a false $H_0$ ). But how do we actually decide whether the evidence from our data is strong enough to reject $H_0$ ? This is where the p-value comes in.

What is a P-value?

The p-value is a probability that measures the strength of evidence against the null hypothesis. Formally:

The p-value is the probability of obtaining test results at least as extreme as the results actually observed, under the assumption that the null hypothesis ( $H_0$ ) is correct.

Think of it this way: you perform your experiment or collect your data, calculate a test statistic (like a t-score or chi-squared value, which we'll cover soon), and then ask: "If the null hypothesis were actually true (e.g., the new model is not better than the old one), how likely would it be to see a result this extreme, or even more extreme, just by random chance?" That likelihood is the p-value.

Interpreting the P-value

The interpretation hinges on how small the p-value is:

Small P-value (e.g., $\le 0.05$ ): Indicates that your observed data is quite unlikely if the null hypothesis is true. This provides strong evidence against the null hypothesis. It suggests something interesting is going on, and the observed effect is likely not just due to random variation.
Large P-value (e.g., $> 0.05$ ): Indicates that your observed data is reasonably likely if the null hypothesis is true. This provides weak evidence against the null hypothesis. It doesn't mean $H_0$ is true, only that your specific test didn't provide enough evidence to reject it.

The Decision Rule: Comparing P-value to Alpha ( $\alpha$ )

To make a formal decision, we compare the p-value to a pre-determined threshold called the significance level, denoted by $\alpha$ (alpha). This $\alpha$ is the probability of making a Type I error that we are willing to tolerate. Common choices for $\alpha$ are 0.05 (5%), 0.01 (1%), or 0.10 (10%).

The decision rule is straightforward:

If $p \le \alpha$ : Reject the null hypothesis ( $H_0$ ). We conclude that there is statistically significant evidence in favor of the alternative hypothesis ( $H_1$ ).
If $p > \alpha$ : Fail to reject the null hypothesis ( $H_0$ ). We conclude that there is not enough statistically significant evidence to support the alternative hypothesis ( $H_1$ ).

Important: Notice we say "fail to reject $H_0$ ", not "accept $H_0$ ". Hypothesis testing is designed to see if there's enough evidence to disprove the null hypothesis, not to prove it's true. A large p-value simply means our test wasn't sensitive enough, or there truly isn't an effect to detect with the current data.

How P-values are Calculated

Calculating the exact p-value involves comparing your calculated test statistic (derived from your sample data) to the known probability distribution of that statistic assuming the null hypothesis is true.

For example, in a t-test, you calculate a t-score. The p-value is the area under the t-distribution curve that is more extreme than your calculated t-score.

The shaded areas represent the p-value (split in two for a two-tailed test). It's the probability of observing a test statistic as extreme as or more extreme than the calculated statistic (vertical dashed lines, Statistic_obs), assuming $H_0$ is true.

The good news is you rarely need to calculate these areas manually. Statistical software and libraries like Python's scipy.stats handle these calculations for you. Your job is to understand the input (your data, your hypotheses) and correctly interpret the output (the p-value).

Avoiding Common Pitfalls

P-values are powerful, but they are also frequently misunderstood. Keep these points in mind:

A p-value is NOT the probability that the null hypothesis is true. It's calculated assuming $H_0$ is true. It tells you about the compatibility of your data with that assumption.
A p-value is NOT the probability that the alternative hypothesis is true.
Statistical significance ( $p \le \alpha$ ) does NOT automatically imply practical significance. A tiny p-value might arise from a very large dataset even if the actual effect size (e.g., the difference in model accuracy) is trivially small and practically irrelevant. Always consider the effect size alongside the p-value.
A large p-value ( $p > \alpha$ ) does NOT prove the null hypothesis is true. It just means you lack sufficient evidence to reject it based on your current data and test.

P-values in Machine Learning Context

In machine learning, you'll encounter p-values when:

Comparing Models: Testing if a new model's performance metric (e.g., accuracy, F1-score) is significantly better than an old model's. $H_0$ might be "The models have equal performance."
Feature Selection: Testing if a predictor variable has a statistically significant relationship with the target variable in a regression model. $H_0$ might be "The coefficient for this feature is zero" (meaning it has no linear effect). This is common in interpreting linear models.
A/B Testing: Determining if a change to a system (e.g., a website layout, a recommendation algorithm) resulted in a statistically significant change in user behavior (e.g., click-through rate). $H_0$ might be "The change had no effect."

Understanding p-values allows you to move beyond just observing differences and make statistically grounded claims about whether those differences are likely real or just due to random chance. In the following sections, we'll look at specific tests (like t-tests and Chi-squared tests) that produce these p-values.

P-values Explained