Disproportionality in UK electoral system

[armb]

A neat interactive demonstration from the BBC:
http://news.bbc.co.uk/1/hi/uk_politics/election_2010/8609989.stm

With the three main parties getting equal shares of the votes, Labour very nearly have a majority of the seats. (Though there are clearly some slightly strange assumptions at the limits, because "Other" still get 24 seats if you set their share to 0.0%, and Northern Ireland only explains 18 of them.)

(via http://www.boingboing.net/2010/04/19/lib-dems-soar-in-uk.html)

Comments

[keristor.livejournal.com]

The others are hard core Plaid Cymru and SNP. Assuming that they don't swing (i.e. you drop the 'other' votes to zero without giving any one of the 3 main parties a noticable boost) they are places which will stay PC/SNP whatever happens (on past performance). Similarly, however, increasing the 'other' vote does very little to the actual number of seats won by 'others, it simply dents the other parties from which they have swung.

[armb.livejournal.com]

My objection was that logically if the model has some seats which (realistically) stay PC/SNP, then it shouldn't let you (unrealistically) drop "Other" to zero, and vice versa.
But that's a detail, and the whole thing is an approximation, obviously. (That doesn't make it foolish, it would just be foolish to read more into it than is there.)