Silver then went and looked at the polling averages for senate races across the country. What he found was at the start of the race, a typical poll would deviate from the average by around 3.5 per cent. This is exactly what should happen, given the basic statistical variation - we know it as “margin of error” - that is an inherent part of any individual poll. But by the eve of the election, that margin had shrunk to 1.7 per cent. Such a change is not statistically credible. Which is why Silver has branded the end of the campaign “CYA – cover your ass time” for the polling companies.