On Resolving Problems with Conditionality and Its Implications for Characterizing Statistical Evidence
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The conditionality principle C plays a key role in attempts to characterize the concept of statistical evidence. The standard version of C considers a model and a derived conditional model, formed by conditioning on an ancillary statistic for the model, together with the data, to be equivalent with respect to their statistical evidence content. This equivalence is considered to hold for any ancillary statistic for the model but creates two problems. First, there can be more than one maximal ancillary in a given context and this leads to C not being an equivalence relation and, as such, calls into question whether C is a proper characterization of statistical evidence. Second, a statistic A can change from ancillary to informative (in its marginal distribution) when another ancillary B changes, from having one known distribution P_B, to having another known distribution Q_B. This means that the stability of ancillarity differs across ancillary statistics and raises the issue of when a statistic can be said to be truly ancillary. It is therefore natural, and practically important, to limit conditioning to the set of ancillaries whose distribution is irrelevant to the ancillary status of any other ancillary statistic. This results in a family of ancillaries for which there is a unique maximal member. This also gives a new principle for inference, the stable conditionality principle, that satisfies the criteria required for any principle whose aim is to characterize statistical evidence.
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