Twelve P-Value Misconceptions

Misconception #1: If P.05, the null hypothesis has only a 5% chance of being true.

Misconception #2: A nonsignificant difference (eg, P .05) means there is no difference between groups.

Misconception #3:  A statistically significant finding is clinically important.

Misconception #4:  Studies with P values on opposite sides of .05 are conflicting.

Misconception #5:  Studies with the same P value provide the same evidence against the null hypothesis.

Misconception #6:  P  .05 means that we have observed data that would occur only 5% of the time under the null hypothesis.

Misconception #7:  P  .05 and P <.05 mean the same thing.

Misconception #8:  P values are properly written as inequalities (eg, “P <.02” when P  .015)

Misconception #9:  P  .05 means that if you reject the null hypothesis, the probability of a type I error is only 5%.

Misconception #10:  With a P  .05 threshold for significance, the chance of a type I error will be 5%.

Misconception #11:  You should use a one-sided P value when you don’t care about a result in one direction, or a difference in that direction is impossible.

Misconception #12:  A scientific conclusion or treatment policy should be based on whether or not the P value is significant.

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