"[...] Given the fact that the nil hypothesis is always false, the rate of Type-I errors is 0%, not 5%, and [...] only Type-II errors can be made, which run typically at about 50% [...] [T]ypically, the sample effect size necessary for significance is notably larger than the actual population effect size and [...] the average of the statistically significant effects is much larger than the actual effect size. The result is that people who do focus on effect sizes end up with a substantial positive bias in their effect size estimation. Furthermore, there is the irony that the "sophisticates" who use procedures to adjust their alpha error for multiple tests (using Bonferroni, Newman-Keuls, etc.) are adjusting for a nonexistent alpha error, thus reduce their power, and, if lucky enough to get a significant result, only end up grossly overestimating the population effect size!"

**J.Cohen, "The Earth is round (p<0.05)".**

A short discussion is maybe needed.

The text above refers to null hypothesis significance tests in psychology. The author explains that when one tests for a correlation between two variables -take them to be number of nobel prize recipients of a country and chocolate consumption for example- the nil hypothesis is that the correlation is exactly zero, while in fact a small, maybe minuscle, but nonzero correlation is bound to exist anyway. So the type-I error rate -the rate at which you mistakenly find a significant correlation with your test- is strictly zero, not 5% (if you do a 95% confidence level test).

The other part of the text is highly amusing to me, because it well applies to particle physics. While in particle physics the null hypothesis is usually true, when it is in fact false a significant bias is usually present in the size of the cross section that is measured for a new state. That is nothing else but the bias that is discussed above.

Finally the "Bonferroni correction" mentioned in the text is the correction for the "trials factor", aka look-elsewhere effect, which accounts for the multiplicity of places where you looked for an effect. In particle physics this is strictly needed, but in studying tiny correlations and given that the nil hypothesis is strictly false, the type-I error rate is zero, so one does not need to correct at all ! Fascinating stuff.

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