Probability that an outcome would occur by chance. P values (probability values) range from 1 (absolutely certain) to 0 (absolutely impossible). A P value equal to or less than 0.05 means that the observed outcome is not likely (≤5%) to be the result of chance.
The probability that a finding has occurred randomly rather than as a result of a treatment or other intervention. In a research study that compares a treated group of patients with a control group exposed only to a placebo, investigators may find that the treated population experienced benefits or suffered more side effects than the controls. Was the observed effect real, or did it occur by chance? The p value of the study helps researchers tell the difference. A p value of 0.5 suggests that there is a 50-50 chance that the findings of the study are significant. A p value of 0.05 (the value customarily used to suggest that research results are statistically significant) means that there is a 5% chance that the results of the study occurred by chance alone. The lower the value, the greater the degree of confidence in the findings: a p value of 0.01, for example, creates more confidence than a p value of 0.05.