What does a P value of 0.05 indicate?

• A. Probability of observing an effect due to chance is 5%
• B. No chance of observing an effect due to chance
• C. 1% chance of an effect
• D. Chance of an effect due to systematic error

Answer: (A)

A P value of 0.05 indicates that there is a 5% probability of observing the effect, or something as extreme as the observed data, purely by chance, under the null hypothesis. This threshold is commonly used in statistical hypothesis testing to assess whether the results are statistically significant. When researchers calculate a P value, they are measuring how compatible their data is with the assumption that there is no actual effect or difference (the null hypothesis).

In this context, a P value of 0.05 suggests that if the null hypothesis were true, there would be a 5% chance of obtaining results as extreme or more extreme than what was actually observed. In practical terms, this means that researchers often use this level as a cutoff to decide whether to reject the null hypothesis. A P value lower than 0.05 typically indicates significant evidence against the null hypothesis, leading researchers to conclude that an effect or association likely exists.

The other options do not accurately reflect the meaning of a P value. For instance, stating there is "no chance" of observing an effect due to chance is incorrect because a P value represents the probability that the observed effect could occur under the null hypothesis. Similarly, a P value does not imply a specific percentage chance