45% support Scottish independence? Ask the 'don't knows'

Scotland's First Minister, Alex Salmond, was on the Andrew Marr show yesterday ahead of this September's referendum on independence. According to him, the polls say the pro-independence campaign has the momentum:

"I mean the polls moved I think from 38 per cent yes in November to 42 per cent the average for February. The most recent ones, 45 per cent, so the Yes Campaign seems to have the momentum"

But we weren't able to find a poll showing 45% support, and neither was BBC presenter Andrew Neil yesterday. However the Yes campaign did speak of such a poll only last week, conducted by Survation. When people were asked "should Scotland be an independent country?" - the wording of the referendum question - 39% said yes and 48% said no, with 13% undecided.

The campaign looked at this poll "on a binary basis" - excluding don't knows - which put the answers at 45% yes and 55% no.

Based on most recent polls, percentage support for Scottish independence has been in the high 30s, with 'don't knows' ranging from 11% to 29% since February. Whether don't knows are factored in or out, the latest poll shows a high level of support for yes compared to recent standards.

YouGov's Anthony Wells has written about how different polling companies deal with 'don't know' responses.