P-value
A measure of the probability that an observed difference could have occurred just by random chance.
What is P-value?
The p-value (probability value) is a number ranging from 0 to 1 that indicates the likelihood of observing the data, or something more extreme, assuming that the null hypothesis is true. It is a foundational concept in frequentist statistics used to determine statistical significance.
Why P-value Matters
It helps researchers decide whether to reject or fail to reject the null hypothesis. A low p-value suggests the data did not occur by random chance.
How to Interpret
If p < 0.05 (the standard alpha level), the result is generally considered statistically significant. If p >= 0.05, there is insufficient evidence to conclude a significant effect.
Example
In a medical trial comparing a new drug to a placebo, a p-value of 0.03 means there is only a 3% probability of seeing the observed difference in recovery times if the drug actually had no effect.
Common Mistakes
- Believing a p-value tells you the probability that the alternative hypothesis is true.
- Assuming a p-value of 0.001 indicates a large or important effect (it only indicates statistical significance, not effect size).
- Using p-hacking to artificially lower the p-value.