In the past, election forecasts have been made on a country-wide basis. A sample of people is carefully selected to represent the population as a whole and they are asked who they will vote for. The percentages for each party are normally pretty good, but how do you convert that to seats in parliament? Just because a party gets x% of the vote doesn’t mean that get the same percentage of seats – a fact well known to the smaller parties. In the past there have been rules of thumb, but these are very crude.
Now there are forecasts by constituency based on a technique called MPR. This stands for “multi-level regression and post-stratification (MRP)”. How does this work?
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Bob Browning's blog 