The extraction of the weak phase $alpha$ from $Btopipi$ decays has been controversial from a statistical point of view, as the frequentist vs. bayesian confrontation shows. We analyse several relevant questions which have not deserved full attention and pervade the extraction of $alpha$. Reparametrization Invariance proves appropriate to understand those issues. We show that some Standard Model inspired parametrizations can be senseless or inadequate if they go beyond the minimal Gronau and London assumptions: the single weak phase $alpha$ just in the $Delta I=3/2$ amplitudes, the isospin relations and experimental data. Beside those analyses, we extract $alpha$ through the use of several adequate parametrizations, showing that there is no relevant discrepancy between frequentist and bayesian results. The most relevant information, in terms of $alpha$, is the exclusion of values around $alphasim pi/4$; this result is valid in the presence of arbitrary New Physics contributions to the $Delta I=1/2$ piece.
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