More on British Polling and Margins of Error

>> Tuesday, March 02, 2010

I promise that this will not be a daily habit of mine, but the Tory +2% lead poll did generate the predictable breathless excitement on these islands.

YouGov's daily tracker indeed moved back towards +6, as anticipated, at Conservative +7. Additionally, a ComRes poll was released yesterday for The Independent which places the Tory lead at +5 (37/32/19). I want to emphasize that the +2 poll released on Sunday, while at the fringes assuming a true value of +6, is still within the margin of error. It was not an outlier in a pedantic understanding of the word, which is a case three standard deviations removed from the mean.

In other words, the findings reported in the Sunday poll were not "wrong". 95% of samples will yield the true value within the error band of +/- 3%, as this was. This is why we have margins of error in the first place.

What we should focus on is not the point estimate itself, but the trend -- and the consistent trend is clearly away from the Tories and towards Labour.

Speaking of which, the +5% Tory lead reflected in The Independent's poll still yields a distribution of seats as Labour 287, Conservative 272, Liberal Democrat 59 -- assuming a uniform national swing.

UPDATE ( 3/3/10): Again as expected, Wednesday's YouGov tracker is rather consistent, at Conservative +5 (C 38 L 33 LD 16). Given the last seven to ten days of polling, this suggests a true value of support around + 6%, and if not precisely 6 (and it isn't) it's slightly below 6%.


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