In praise of the ‘Don’t know’ voter

I am scraping the edges of my memory here, but I am fairly sure that opinion polls in my childhood (for the elections of 1970, 1974 and 1979) quoted four percentages: Conservative, Labour, Liberal and ‘Undecided’. Nowadays no figure is quoted for ‘Don’t knows’, and party support is contrived to add up to 100 per cent. Undecided respondents are variously treated according to each polling company’s methodology: a few ignore them completely; others apply a supplementary question such as ‘Which way would you vote if voting were compulsory?’ Their answer to this may be statistically downweighted, but it will still be added to the total for one party or another, rather than being listed under ‘Don’t know’.

This seems a very bad practice. If a research company were presenting consumer reaction to a new shampoo, I would expect to see a percentage figure for the undecided. Not listing such people separately creates a false impression of certainty.