The set of final polls from each pollster published in three recent UK referendum campaigns (Welsh devolution 2011, electoral reform 2011 and Scottish independence 2014) consistently under-estimated the actual vote for the ‘status quo’ option.
Taking the data from the excellent Elections in Wales site, we get:
- Welsh devolution referendum 2011: poll average 31% no, actual 37% no.
- Electoral reform (AV) 2011: poll average 64% no, actual 68% no.
- Scottish independence 2014: 53% no, actual 55% no.
So far, this pattern has not embarrassed pollsters are they’ve got the winner right and in some cases the final fieldwork from a pollster was a good few days before polling day.
But the overall pattern is consistent, and fits with wider international experience – the public usually moves towards the status quo option as polling day nears.