Fox and why focus groups fail
The very fact that the focus group was on live TV corrupted the Fox focus group because perspective voters would not be making decisions in that kind of situation. Being on TV has a huge influence on someone if you are asking for an opinion.
In addition, the environment was one where very strong individuals could pressure weaker members into like positions, and if you watched you saw this play out. Not only were there far more people in support of Trump at the beginning than appeared to be the case in the broad audience, but the percentage change of supporters was unreasonably large.
Once someone takes a position it is hard to get him to change it by simply watching a candidate talk because, with confirmation bias, they'll generally hear what they want to hear particularly early on. Yet the focus group appeared to swing solidly against Trump by the end even though Trump's poll numbers didn't shift nearly as much.
In short, at the beginning there were likely folks saying they supported Trump who really didn't, and at the end there were folks going with the crowd who wouldn't have without pressure. So you had too much support up front and then too little at the end with the result being a massively inaccurate representation of what had and eventually did happen. In the end, the focus group was worthless other than as an attack vehicle against Trump, which backfired.
The analyst appeared to be pushing his group against Trump and later Trump indicated the two had history and it wasn't good history. The focus group moderator/analyst has to be unbiased otherwise they will corrupt the results either by influencing the focus group and/or by misinterpreting the results.
Now in the case of Fox it did look like it might be manipulating the results in order to influence the population against Trump, which Trump himself seemed to pick up and there is science here because false analysis results, if presented well, can change minds. And it is possible that some of Trump's slight initial fall in the polls could have resulted from this.
So you can use a focus group as a competitive weapon but, to be effective, the population has to identify with the group and find the reasons for their positions compelling, both of which require a lot of reach for a large population (which FOX had). You also have to bet the other side won't be successful in proving your focus group or survey is false, which would destroy your credibility. But there is a big difference between being a representative and being a manipulation tool, and a real danger of forgetting which you are supposed to be.
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