P-Value Calculator

A p-value is the probability of obtaining an outcome at least as extreme as the one observed, assuming that the null hypothesis is true. It quantifies the strength of evidence against the null hypothesis.

• Small p-values (e.g., < 0.05): Strong evidence against the null hypothesis.
• Large p-values: Little evidence against the null hypothesis.
• Interpretation: A p-value does not tell you the probability that the null hypothesis is true.

The formula to calculate the p-value depends on the type of test and the distribution. In general, the p-value is calculated as:

• Left-tailed test: $P(X \leq x)$
• Right-tailed test: $P(X \geq x)$
• Two-tailed test: $2 \times P(X \geq |x|)$

Where $X$ is the random variable and $x$ is the test statistic. For example, for a Z-test, given the test statistic $z = 1.23$, here is how you calculate the p-value for a two-tailed test:

In hypothesis testing, the p-value is compared to the significance level (α) to determine the outcome of the test. Here's how to interpret p-values:

• p-value < α: Reject the null hypothesis.
• p-value >= α: Fail to reject the null hypothesis.

In our last example, ince the p-value (0.2186) is greater than the significance level (0.05), we fail to reject the null hypothesis.

In R, you can use the pnorm() function for Z-tests, the pt() function for t-tests, pf() for F-tests, and pchisq() for Chi-Square tests to calculate p-values. The letter 'p' in these functions stands for the cumulative probability. Here are some examples of how to calculate p-values in R:

# Calculate p-value for a z-test (two-tailed)
test_statistic <- 1.96
p_value <- 2 * (1 - pnorm(abs(test_statistic)))
cat("P-value:", round(p_value, 4)) # Output: P-value: 0.05

# For a t-test with df = 10:
test_statistic <- 2.1
p_value <- 2 * (1 - pt(abs(test_statistic), df = 10))
cat("P-value:", round(p_value, 4)) # Output: P-value: 0.0621

# For a chi-square test with df = 5:
test_statistic <- 10.25
p_value <- 1 - pchisq(test_statistic, df = 5)
cat("P-value:", round(p_value, 4)) # Output: P-value: 0.0685

# For a F-test with df1 = 3 and df2 = 5:
test_statistic <- 4.5
p_value <- 1 - pf(test_statistic, df1 = 3, df2 = 5)
cat("P-value:", round(p_value, 4)) # P-value: 0.0695

In Python, you can use the scipy.stats module to calculate p-values. Use stats.norm for Z-tests, stats.t for t-tests, stats.f for F-tests, and stats.chi2 for Chi-Square tests. Here are some examples of how to calculate p-values in Python:

import scipy.stats as stats

# Calculate p-value for a z-test (two-tailed)
test_statistic = 1.96
p_value = 2 * (1 - stats.norm.cdf(abs(test_statistic)))
print("P-value:", round(p_value, 4))  # Output: P-value: 0.05

# For a t-test with df = 10:
test_statistic = 2.1
p_value = 2 * (1 - stats.t.cdf(abs(test_statistic), df=10))
print("P-value:", round(p_value, 4))  # Output: P-value: 0.0621

# For a chi-square test with df = 5:
test_statistic = 10.25
p_value = 1 - stats.chi2.cdf(test_statistic, df=5)
print("P-value:", round(p_value, 4))  # Output: P-value: 0.0685

# For a F-test with df1 = 3 and df2 = 5:
test_statistic = 4.5
p_value = 1 - stats.f.cdf(test_statistic, dfn=3, dfd=5)
print("P-value:", round(p_value, 4))  # Output: P-value: 0.0695