The outcome of voting under PR-STV is proportional in party terms within a single election to the collective preference of voters, assuming voters have ranked their real preferences and vote along strict party lines. However, due to other voting mechanisms usually used in conjunction with STV, such as a district or constituency system, an election using STV may not guarantee proportionality across all districts put together. Differential turnout across districts, for example, may alter the impact of individual votes in different constituencies, and when combined with rounding errors associated with a finite number of winners in each constituency the election as a whole may throw up anomalous results from a purely party perspective.