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Two-party swing is the average of one party's gain and the other's loss, which is why it is half the sum of the two changes. Halving avoids double-counting the same transferred voters and gives the single figure used to compare constituencies.
Butler two-party swing
Swing = [(B current - B previous) - (A current - A previous)] / 2
Indicative estimate only. Fees, entitlements, limits and formulas vary by jurisdiction, statute, policy wording and the facts of the case. This is not legal, tax, insurance or financial advice — confirm with a qualified professional or the relevant authority.
Because a voter moving from A to B shows up twice — as a loss for A and a gain for B. Averaging the two counts the movement once.
Total vote swing (Steed swing) uses only the change in one party's share, which is more appropriate in genuinely multi-party contests.