What is the meaning of saying that random variables {X1, …, Xn} (such as aptitude scores or hypothetical response time components), not necessarily stochastically independent, are selectively influenced respectively by subsets {Γ1, …, Γn} of a factor set Φ upon which the joint distribution of {X1, …, Xn} is known to depend? One possible meaning of this statement, termed conditionally selective influence, is completely characterized in Dzhafarov (1999, Journal of Mathematical Psychology, 43, 123–157). The present paper focuses on another meaning, termed unconditionally selective influence. It occurs when two requirements are met. First, for i=1, …, n, the factor subset Γi is the set of all factors that effectively change the marginal distribution of Xi. Second, if {X1, …, Xn} are transformed so that all marginal distributions become the same (e.g., standard uniform or standard normal), the transformed variables are representable as well-behaved functions of the corresponding factor subsets {Γ1, …, Γn} and of some common set of sources of randomness whose distribution does not depend on any factors. Under the constraint that the factor subsets {Γ1, …, Γn} are disjoint, this paper establishes the necessary and sufficient structure of the joint distribution of {X1, …, Xn} under which they are unconditionally selectively influenced by {Γ1, …, Γn}. The unconditionally selective influence has two desirable properties, uniqueness and nestedness: {X1, …, Xn} cannot be influenced selectively by more than one partition {Γ1, …, Γn} of the factor set Φ, and the components of any subvector of {X1, …, Xn} are selectively influenced by the components of the corresponding subpartition of {Γ1, …, Γn}.
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