The P value is 0.0001 because the probability under H 0 of observing an observation as extreme as this is 0.0001. We have every right to reject H 0 at the 0.05, 0.01, or even the 0.001 level of significance. Even though we can reject H 0 at the usual levels of significance, common sense says that the null hypothesis is more likely to be true than the alternative hypothesis. I was asked recently why confidence intervals were common in the medical literature but not in other fields. Many areas of investigation summarize their experiments in scales and indices that often lack an operational interpretation.
Some scales are the sum of positive responses to items on a questionnaire.
Others are composites of related but different components. Those scoring higher are different from those scoring lower, but it's often not clear what a 1 or 10 unit difference means period, let alone in terms of practical importance.