Candidate of the Democratic Party vs Trump: Where are we now?

 Hi,

 

I waited for things to stabilise before publishing new analyses. In this blog message, I analyze the polls conducted since June 14, two weeks before the debate between Joe Biden and Donald Trump, up until the last polls published this morning.

Here are the trends in voting intention for the candidate of the Democratic Party (Biden until July 21st and Harris afterwards) and for Donald Trump, candidate of the Republican Party. The following graph shows that,support for Donald Trump placed him in very good position before and after the debate, However since then, and particularly after Joe Biden's withdrawal and hi eventual replacement by Kamala Harris, the situation changed completely. Most polls that have been published since Harris took the lead as the possible candidate for the Democratic party favor her. According to the polls, she would be at about 52% right now. However, some polls still see Trump as leading. The next graph will show the characteristics of those polls.

 

 

The following graph presents the poll results according to mode combination. Because we need to have enough polls in each combination for our purposes, we categorized the polls in two groups, polls using web opt-in only and polls using multiple modes. There are much more polls using web opt-in conducted recently than polls using a combination of modes. It seems rather clear that, although both types of polls estimate a clear increase in support for Kamala Harris. However, the web opt-in polls, as in the 2016 and 2020 US presidential elections, estimate the support for the democratic candidate higher than the other polls. And in 2020, the "other polls" were right. Of course, meanwhile pollsters have worked on improving their methodology. Nonetheless, the difference is persistent. This difference may have several causes, not necessarily related to the quality of the polls but it is possible that the 2024 election will give us another test of mode effects.

 

In conclusion, everybody should be careful in the conclusions they draw from polls. The campaign is just starting.

 

** Methodology: 

First, I take only one poll by pollster by period. Many pollsters ask two questions, one where they present all the candidates and one where they force respondents to choose between the two main candidates. I chose to always take the poll that mentioned all the candidates because it seemed more comparable to the other polls.  

Second, the trends are calculated using local regression. I always used a parameter of .65 (the percentage of points used for adjustment) for the analysis, because it was the parameter most recommended in the literature. It gives smoother trends. However, it did not seem appropriate in a situation with much movement. I therefore changed it to 0.5 which gives trends that are more sensitive to movement.

 

Support for Biden and Trump on the eve of the first debate

 

Hi,

A short message on the polls of the 2024 US presidential election. I decide to follow the polls early on this time since we already know who are the candidates. This will allow me to better follow what is happening and the quality of the polls. I will introduce two graphs, one on the global trends in vote intention and a second one on the trends according to mode of administration and sampling sources combinations.

 

Trends in voting intentions since January 1st

 This first graph portrays the trends in the proportion of support for each main candidate on what is called the 2-party share, that is, the sum of voting intentions for Trump and Biden. This is the only comparable metric. It is important to note that pollsters do not always publish comparable estimates. Some include the small candidates, other don't. Some group together the small candidates and the "Don't know". Some ask the question twice, once with all the candidates and the second time with only Trump and Biden (we selected only the first question for analysis). Most pollsters do not exclude the "don't know", a proportion that differs substantially between pollsters. In short, it is essential to compare using the 2-party share.

 The graph illustrates what happened since January 1st.  As you can see, the gap between Trump and Biden seems to have decreased. Trump is still ahead but by a very slight margin. However, we see that some polls differ substantially from the others: Some place Trump way ahead, other put Biden ahead of Trump. Be reassured, these "outliers" do not substantially influence the trends presented here given the method used (local regression). However, is there a possible explanation?

 

Trends according to mode-source combinations

The following picture shows the same data for two types of mode-source combination. One type of combination is Web Opt-in panel only. The other type is Multiple modes, a combination that includes at least partially the use of random or quasi-random sampling. I have shown here that mode-source combinations had a substantial impact on estimates in the US 2020 election. The question is whether the impact is present from the beginning. It was not that evident until recently.

There are three types of mode-source combinations, described in the article referred to above. The third type is "one mode, random or quasi-random". It includes telephone polls. There are not enough polls in this category for analysis right now. Therefore, these polls are not included. The Activote polls are not included either because we cannot categorize them (see below).

The graph shows the trends in support for Biden portrayed by the two types of mode-source combination. As during the campaign in 2020, web opt-in polls seem, on average, not to detect any movement, besides a small linear increase in support for Biden, while polls using multiple modes do. They show an increase followed by a recent decrease. Who is right?

 

 

The recent decrease portrayed by polls using multiple modes may be partly due to some specific practices. For example, the "outlier" putting Biden under 45% recently is a Rasmussen Reports poll. I decided to check and realized that the question on voting intention followed 10 different questions on immigration policies and preferences, a topic that is quite polarising. This practice may have biased the estimate in favor of Trump. However, even if I drop this poll, the difference between the two combinations are still present. 

This being said, Rasmussen published its questionnaire, a practice not always present. Whitout the questionnaire, we can't know the rank of the question on vote intention and whether the questions asked before it may bias the estimates. My understanding is that the current practice is to ask about voting intention at the beginning of the questionnaire and not ask questions that are polarising before the vote intention question.

A second pollster used a methodology that is quite disturbing. Activote seems to rely only on people  recording their vote intention on a free application available on phones and computers. The application is always open so that people can answer when they want. Activote publishes estimates when they feel they have enough answers (200 for state polls). The polls are conducted over 10 to 15 days. Activote does not say whether they check for people answering multiple times. Their estimates are quite stable, around 48-49% for all polls.

Conclusion

Biden and Trump are close, very close, everybody knows. But we also have to be aware that the way the polls are conducted may have an impact on the estimates that are published. In addition, when looking at polls, it is important to check all the information that we may have -- or not -- on the methodology, including the questionnaire. As a general rule, the less information you have on a poll, the less trusty the poll is. But sometimes, the information provided also leads to doubt about the numbers published. However, it still gives you some relevant information.