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The Standard Note you quote is just Adam Mellows-Facer's opinion. I think I sent in several corrections to Standard Notes and Research Papers over the years then I just got bored with it. To take just one example, the SN refers to the 1935-1945 swing as being meaningless because ""National Labour candidates contributed 1.6% to the National total in 1935"". True to say that but totally irrelevant: most voters in 1935 understood National Labour to be in effect Conservative, and it was only the Macdonald family which thought of it as being Labour. What the SN says when giving examples is no more definitive than a newspaper article and I absolutely refute that it is 'authoritative'.

The reason why swing is used for Conservative and Labour only and not for any other parties is that it doesn't work properly for others. The calculation is only any use if you can take the result and apply it elsewhere to compare and contrast, and that only works for Conservative and Labour in practice. Look at the distribution of the so-called ""Conservative - Liberal Democrat swing' and it's completely random.  | Talk "