samedi 22 avril 2017
Tomorrow, the French presidential election
First a word to say that there is a new feature in this blog, for this election. This blog is a collaboration with P.J. Fournier (Qc125.com), which allows for a combination of competence.
And so, today’s topic, what should we expect for the French presidential election tomorrow?
Let us examine first the polls published during the last weeks. This first graph show change in support for the five main candidates since the beginning of March. It shows what other researchers, pollsters and media alike have shown. Support for Macron and Le Pen has varied a little since the beginning of the campaign. Support for Macron is quite stable. Meanwhile, support for Le Pen decreases somewhat. Support for Fillon has been stable lately while Hamon and Mélenchon exchange support. At the end, Mélenchon is now tied with Fillon at 20%. These estimates are dependent upon all the polls published since the beginning of March.
If we use only the polls conducted since the beginning of April, the analysis is less dependent upon older polls. Is the portrait different? Not really. The conclusions are the same. All the polls show Macron slightly ahead of Le Pen, with Fillon and Mélenchon tied in 3rd and 4th place.
And what about the margin of error?
If we take the margin of error of each poll, we could conclude, as some have done, that « everything can happen”, this because support for the four main candidates are usually within the margin of error for individual polls. This interpretation is not adequate. The margin of error for the all the polls combined is much smaller. In a way, it is as if we could combine all the samples or if we took into account the fact that the current estimations are dependent upon the previous ones. It is not as if « anything » can happen. Since the method used in the preceding graphs does not allow to show the margin of error of the estimation of the regression lines, the following graph will allow to show this in a more evident way.
Since there has been lots of movement in the two last weeks, it is possible to estimate the margin of error for all the polls published during that period, in a way as if they had all been conducted at the same time. However it gives more weight to the last polls (using a squared weighting) in order to compensate. It is a conservative estimation of the margin of error. The following graph allows for a visual portrait of the confidence intervals for each candidate’s support. It takes into account all the polls conducted from April 8 to 21. The intervals are not large since there is not much variation between the estimates of the various pollsters because of the methodology they use (see later on in this post).
What can we conclude? If the polls are reliable, it is impossible to be sure who, between Macron and Le Pen, will finish first because statistically they get equal support (the confidence intervals overlap). The same thing happens with Fillon and Mélenchon. However, the graph also shows that the confidence intervals for Fillon and Le Pen slightly overlap. Le Pen could get a score as low as 20.7% while Fillon could go as high as 21%, i.e. he could finish second. On the contrary, Mélenchon is significantly lower than all the other main candidates. In short, if we rely on polls, the two who are most likely to finish first are Macron and Le Pen but the possibility of a Macron-Fillon 2nd round also exists. We have tried other hypotheses using different periods and weights and we get similar results.
Are polls reliable?
This is « the question ». In order to answer this question, one has two rely on two types of information, i.e. the methodological ones and history.
First, methodological information. What is interesting in the French situation is that the pollsters have to file methodological information and their data with the Commission des sondages, a government body. The Commission’s experts can check the data and decide whether the estimates match the data for each poll. The methodological “Notices” are available for everybody to consult on the Commission’s web site here: http://www.commission-des-sondages.fr/notices/.
One author of this blog, Claire Durand, had examined these files for the 2002 presidential election (see: https://academic.oup.com/poq/article/68/4/602/1884181/The-Polls-in-the-2002-French-Presidential-Election#29033826https://doi.org/10.1093/ijpor/edn029
In conclusion, unless there is a catastrophe like in 2002, which is very unlikely given the methodological information provided, the highest probability is Macron – Le Pen for the 2nd round. All the analyses lead to this conclusion. A poll conducted in the middle of last week has shown Le Pen, Fillon and Mélenchon tied but the next polls did not show similar estimates. In fact, the most recent polls all showed increase support for Macron. The second possibility would be Macron-Fillon for the 2nd round. It is not likely but it cannot be totally excluded. Why not a Macron-Mélenchon? In addition to what has already been mentioned, we should add that the Brexit reminded us that “old” people win consultations, and “old” people support Fillon more than Mélenchon. Is it possible that the polls go wrong? In such an election, it is possible, but it is unlikely.