he standard level of significance used to justify a claim of a statistically significant effect is 0.05. For better or worse, the term statistically significant has become synonymous with P0.05.
There are many theories and stories to account for the use of P=0.05 to denote statistical significance. All of them trace the practice back to the influence of R.A. Fisher. In 1914, Karl Pearson published his Tables for Statisticians & Biometricians. For each distribution, Pearson gave the value of P for a series of values of the random variable. When Fisher published Statistical Methods for Research Workers (SMRW) in 1925, he included tables that gave the value of the random variable for specially selected values of P. SMRW was a major influence through the 1950s. The same approach was taken for Fisher's Statistical Tables for Biological, Agricultural, and Medical Research, published in 1938 with Frank Yates. Even today, Fisher's tables are widely reproduced in standard statistical texts.
