I was looking for polling organizations and stumbled across this:
http://en.wikipedia.org/wiki/Shy_Tory_FactorShy Tory Factor
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Shy Tory Factor is a name given by British Opinion polling companies to a phenomenon observed in the 1990s whereby the share of the vote won by the Conservative Party in elections was substantially higher than the proportion of people in opinion polls who said they would vote for the party. The Conservative Party is often referred to by the name of its predecessor party, 'the Tory party'.
In the 1992 general election, the final opinion polls gave the Conservatives between 38% and 39% of the vote, about 1% behind Labour. In the final results, the Conservatives had a lead of 7.6% over Labour. As a result of this failure to 'predict' the result, the Market Research Society held an inquiry into the reasons why the polls had been so much at variance with actual public opinion. The report found that 2% of the 8.5% error in the party lead could be explained by differential refusals to be interviewed by Conservative voters; it cited as evidence for this factor the fact that exit polls on election day also underestimated the Conservative lead, when they could not be affected by sampling error.
After the election, most opinion pollsters altered their methodology to try to correct for the observed behaviour of the electorate being less likely to admit to voting Conservative. The methods varied for different companies. Some asked their interviewees how they had voted at the previous election, and assumed that those who had voted Conservative before but were now unsure or refusing to answer would return to the party. Others weighted their panel so that their past vote was exactly in line with the actual result of the election. For a time, opinion poll results were published both for unadjusted and adjusted methods.