Máy tính giá trị p
The detailed guide below is currently available in English.
What a p-value actually measures
A p-value is the probability of seeing a statistic at least as extreme as yours if the null hypothesis were true. The two-tailed value counts extremes in both directions — appropriate when a difference could go either way — while the one-tailed value counts only the direction you specified in advance. This calculator evaluates the real distributions numerically: the normal tail via a rational approximation to the error function, the t tail through the regularized incomplete beta function, and the chi-square tail through the upper incomplete gamma function. Convention treats p below 0.05 as statistically notable, but the threshold is a custom, not a law.
Câu hỏi thường gặp
Should I report the one-tailed or two-tailed p-value?
Default to two-tailed. Reserve one-tailed tests for the rare case where only one direction of effect could possibly matter — and decide that before seeing the data, never after.
How many degrees of freedom do I use?
For a one-sample t-test use n − 1; for a two-sample test roughly n₁ + n₂ − 2; for a chi-square goodness-of-fit test use categories − 1, and for independence use (rows − 1) × (columns − 1).