Pool for single-member district tiers in elections to the upper house. The Pool variables measure the extent to which votes among candidates from the same party are shared. The variables equal (in order of increasing personal vote incentives): (0) where pooling of votes occurs across all candidates in a party in a district; (1) where pooling of votes occurs across some, but not all, candidates in a party in a district, or, where there is vote pooling across all candidates in a party in a district, but where the average district accounts for 5% or less of a legislature's membership; (2) where no pooling of votes occurs across candidates in a party (including single-member districts).