This is the last month before the referendum on Scottish independence, an appropriate moment to compare the current situation in Scotland and the situation that prevailed in 1995 in Quebec.
First, let's examine the current situation in Scotland. With 49 polls published in 1995, one month to go, the likely situation right now is a gap of more than 10 points in favour of the No (12 points if we distribute the non-disclosers proportionally), as the following graphs show. Some have maintained that a rise in support for the Yes side have occurred last week. This is not obvious right now. As we can see, there is variation in the estimates published by the different pollsters. If we take that into account, we are still in a situation of no change in either direction. The impact of the debate held on August 5 may have been to reduce the proportion of non-disclosers (including mostly don't knows).
In evaluating the situation, we have to take into account the fact that half the polls, i.e., those conducted using opt-in panels by ICM, Survation and Panelbase, put support for the Yes side 4.5 points higher on average than the other polls (*see methodological notice at the end). If we look at all the elections where opt-in panels were compared to election results, we may conclude that these polls likely overestimate support for the Yes side. Let us now compare this situation with what happened in Quebec during a similar period to see if it can teach us something.
What was the situation in Quebec during about the same period? I kept all the polls -- including those conducted by academics, governmental agencies and political parties -- conducted between February 15, 1995 and Referendum Day, October 31th, 1995. I kept the results only when the question asked about vote for Sovereignty with an association with the rest of Canada. Before June 12 (first vertical line on the graph), when an agreement between Parti Québécois, Action Démocratique du Québec and Bloc Québécois -- called the 3-party agreement -- was signed, it was not clear what the question would be and whether a partnership with the rest of Canada would be part of it. Therefore, there are not many polls asking about this option before June 12. I have shown elsewhere that support differs according to the constitutional option -- independence, separation, sovereignty, sovereignty with partnership -- and that only the last option managed to reach majority support during some periods.
The first graph shows the change in support from February 15, 1995 to Referendum Day, including the non-disclosers. It shows that support for sovereignty with an association with the rest of Canada managed to get the support of a plurality of Quebeckers until two months before referendum Day. However, after the official launching of the campaign on September 7 (second vertical line on the graph), Support for the Yes side decreased to 40% and the proportion of non-disclosers increased until the last month. Then support for the Yes side went back up until referendum Day. The final two months are characterized by a roller-coaster type of movement and an increase in the proportion of non-disclosers (including don't knows).
The second graph shows the same information after proportional attribution of non-disclosers (**see methodological notice at the end). Again, it shows a situation favorable to the Yes side, up until the launching of the campaign on September 7 and a roller-coaster ride afterwards.
Let's now focus on the last stretch. The third graph shows that, when the campaign was launched, the two sides were at par. Support for the Yes side went down during the first month so that one month before referendum Day, support for the Yes side was five to six points lower than support for the No. However, support for the Yes side went back up to finish 6 points higher than support for the No. However, as we can see, with proportional attribution of non-disclosers, support for the Yes was seriously overestimated. The referendum ended with 49.5% Yes and 50.5% No. With realistic (*) attribution of non-disclosers, the prediction of the results was almost perfect.
Conclusion - Scotland vs Quebec
There are a number of differences between the Quebec 1995 campaign and the Scottish 2014 campaign. For instance, the Quebec question pertained to sovereignty accompanied by an offer of partnership to the rest of Canada and it was officially revealed less than two months before referendum Day. Independence per se (the Scottish option) never managed to get the support of a majority of Quebeckers in the polls.
In terms of poll results, the Quebec question managed to get the support of a short majority of Quebeckers during most of the period, which has not been the case in Scotland. When the Quebec Yes side increased its support during the last month in 1995, it was regaining support from people who had already been inclined to vote Yes at some point during the preceding months. Scotland's Yes side cannot rely on this possibility. It has to convince voters to change a position they held for months. Everything is still possible but, since no movement in support has occurred since March, the Scottish Yes side needs a very major event in order to gain the support of a majority of Scottish people. As the Quebec situation shows, it is possible to reverse the trend. In Quebec, a change in strategy -- i.e. the nomination of Lucien Bouchard as negociator in chief on October 7th -- seemed to have helped close the gap with the No side. However, Scotland's Yes side would probably need more than that. The current situation in Scotland resembles more to what happened in the first Quebec referendum in 1980 that ended with 59.6 for the No side than to what happened in the second referendum in 1995.
P.S. I will be away from August 24 to September 8. I will be back to monitor the last stretch.
* The other pollsters are TNS-BRMB (face-to-face), Ipsos-Mori (telephone) and YouGov (opt-in panel). These three pollsters had similar estimates since January. Note however that You Gov has just changed some features of its methodology (see here). It announced that it now includes respondents that are 16 and 17 year old (which means that they were not included in preceding polls!!) and that it modified its weighting scheme to take into account country of birth. YouGov states that its results for the last poll are not comparable with previous results and that the proportion of support for Yes Scotland is a bit higher with these new features.
** At that time, in Quebec, media and researchers alike tended to use a "realistic" attribution of non-disclosers that gave 75% of them to the No side. This practice tended to put both sides almost equal and gave a very good prediction of the final result. Some Media published three figures, i.e., voting intention before attribution of non-disclosers, with proportional attribution and with "realistic" attribution.