Some people are puzzled about what election commentators mean when they talk about "swing". Why do we say that there has been a swing of 4-5% from Labour to Conservative when the main movement so far has been from Labour to Lib Dem?
The answer is that "swing" is just a simple way of describing the net effect of all sorts of complex movements between the parties. It was devised in an era of two-party politics, when over 90% of people voted Labour or Conservatives and there were lots of "straight fights".
Swing can be defined as the average of the percentage point loss for any party and the percentage point gain for another. The main movement may be between Lab and Lib Dem - but that is of interest only in determining a few seats. We tend to think about swing between Labour and Conservative because that determines the parliamentary majority.
