Definition:Pairwise Balanced Design

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Let $\struct {X, \BB}$ be a design.

Let $\card X \ge 2$.

Then $\struct {X, \BB}$ is a pairwise balanced design, abbreviated PBD, if and only if every two distinct points in $X$ belong to exactly $\lambda$ blocks, for some constant $\lambda > 0$.

That is, if and only if any pair of distinct points appears in the same number of blocks as any other pair of distinct points.