A Dirty Dozen: Twelve P-Value Misconceptions 

By Goodman S

Date: 01/2008
Journal: Seminar of Hematology

Summary :

  • Twelve P- Value Misconceptions: If P=.05, the null hypothesis has only a 5% chance of being true.​
  • A non significant difference (eg , p.>or =.05) means there is no different between groups.​
  • A statically significant finding is clinically important.​
  • Studies with p values on opposite sides of .05 are conflicting .​
  • Studies with the same p values provide the same evidence against the null hypothesis.​
  • P=.05 means that we have observed data that would occur only 5% of the time under the null hypothesis.​
  • P=.05 and p<or =.05 mean the same thing.​
  • P values are properly written as inequalities (e g, “p<or = ,02”when p=,015)​
  • P=.05 means that if you reject the null hypothesis, the probability of a type I error is only 5%.​
  • With a p =,05 threshold for significance, the chance of a type I error will be 5%.​
  • You should use a one- sided p value when you do not care about a result in one direction, or a difference in that direction is impossible.​
  • A scientific conclusion or treatment policy should be based on whether or not p value is significant.