On this topic, there's a very interesting new book: "The Book of Why: The New Science of Cause and Effect" by Judea Pearl and Dana Mackenzie. I haven't finished it but I can recommend it. —Dan Henry Baker wrote: ----- I'm not an expert in statistical analysis, and I'm having a hard time reconciling all of the features of the modern scientific method. In particular, the usual process goes something like the following: 1. A scientist observes some phenomena and detects some correlations between observations of type A and observations of type B. 2. The scientist *hypothesizes* some causality among the observations. 3. The scientist designs some *experiment* to try to determine causality. 4. But since the scientist already has preconceived notions about the causality, he/she is not the appropriate person to *perform* the experiment; better to *double blind* the study and have someone *completely ignorant of the experimental design* perform the experiment on subjects (in the case of animate subjects) who are also *completely ignorant of the experimental design*. 5. The data from the experiment can be analyzed by yet another party who is *completely ignorant of the experimental design*, so that his/her biases cannot affect the analysis. In a perfect, causal world, such a proper experiment should show causality if and only if the causality exists. In particular, a "proper" experiment should have N large enough so that the probability of false positives and false negatives are unbelievably small. Here's my problem: Scientists have been accused of *fitting to the facts* -- i.e., coming up with hypotheses *after the experiment* that match the experimental results. Furthermore, some have recommended that all such "a posteriori" papers be firmly rejected as scientific fraud. My question is: "how can our universe possibly tell whether the hypothesis was suggested before or after the experiment?" In a classically causal universe, the timing of the hypothesis and the timing of the experiment should make no difference, because the mental state of the scientist can't possibly affect the results of the experiment. If an experiment is indeed performed completely blind by disinterested third parties, why should anyone care how or *when* the hypothesis was obtained? -----