What was the probability that Barack Obama would win the US
election?  On the face of it this seems an odd question. After
all, he won. But before the election it was uncertain whether
Obama would win, and probability is the way that uncertainty is
quantified, so maybe it is reasonable to ask what that
probability was.  We know that there were betting odds – a
betting exchange such as Intrade allows people both to accept or
make bets and so converges, at any point in time, to a certain
set of odds at which people are willing to be either the better
or the bookmaker. This prediction market provides a
‘probability’ on Obama winning that kept changing for the year
before the election – this is shown in Figure 1 with some of the
main events of the year marked in.  Obama Odds  'Probabilities'
of Obama and McCain winning the 2008 US Presidential Election
We can see how the betting odds changed in reponse to events.
Before getting the nomination by the Democratic party, Obama had
to compete with other candidates, p