If it were as easy as you make out to predict the result based on past voting percentages and a biased electoral system, there'd be little point in bothering with a campaign.
I don't think it's easy to predict, because there are still three possible outcomes: Labour majority, Labour minority or Tory minority. All I'm predicting is no Tory majority, although if Ashcroft reveals on Saturday that the Tories have significantly closed the gap in the marginals from his poll last summer which showed Labour 14pts ahead (twice their national lead) and if the Scots vote for independence (currently 58-42% in favour of No with the gap now widening), then I might revise that.
As for the 3rd and 4th parties, the polls have consistently pointed to one pattern and that is Labour benefitting massively. The key is the 2010 Lib Dem voters. Contrary to popular belief, they actually did very well in the last election with 6.8m votes, and between 25 and 40% of those have told the polls every week since 2010 that they will vote Labour. That is a huge number and it has to reduce significantly to deny Labour. Then there's the 2010 Cons who plan to vote UKIP. That number isn't reducing, but worryingly for Labour, UKIP are now picking up their voters. That might turn into the biggest battleground, but there's little that Crosby and the papers can do about LDs defecting to Labour, and it's that group, above all, which decides whether Miliband gets the key to no10.
You're right that 1992 wasn't predictable, but the main reason for that is that the poll companies got the figures wrong. Their methods were proven to be flawed. They overstated Labour's lead by not taking into account what's now referred to as Shy Tory Factor so it's a myth that millions of people changed their mind at the last minute. They've since altered how they deal with the 'Don't Know' responses to take this into account, and they've also introduced weighted and unweighted samples. If anything, the polls are now overstating the Conservative position, although if that's true the margin of error will be far less. The other pattern that is different is that voters tend not to move from Conservative to Labour or vice versa. They go to another party now, so swings are less pronounced.
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