Accuracy of election polls There are a number of questions about election polls and margin of error that have been covered in CrossValidated over the years, including one regarding the 2016 US election polls. That one however only addresses validity based on members of a particular poll knowing each other. 
In so many cases the state polls showed Clinton beating Trump above the margin of error.  Yet states like Wisconsin, Pennsylvania, Florida and Michigan were predicted to be won by Clinton but subject to recount all went to Trump. Of course there are many factors that could affect the accuracy of the polls and the validity/lack of validity of the margin of error.  Among those that I can think of are:


*

*difficulty selecting a random sample of likely voters

*the election was volatile and hence present opinion may not reflect future opinion

*selection bias could exist
Which of these possibilities do you think caused the inaccuracy of the polls or do you have another explanation?
What assumptions about statistical surveys are most commonly violated.
 A: *

*It seems highly improbable that sampling variation would cause this problem. How could it affect so many polls then? And wouldn't we have seen this before in other election polls in the past? 

*It's possible that some voters changed their opinion in the end, almost as they were in the election booth. One possible behavioral effect is that people wanted to "stick it to the system". In that case, given that they had seen Trump being behind in all polls, there would be almost no cost to them voting for him. Maybe they wanted a close election since this would signal to the establishment that while they wanted Hillary, something was really wrong since they were so close to electing Trump. If many felt like this, maybe this explains the last-minute swing. One might conjecture a similar effect in the Brexit vote, where all the polls were also off.  

*One possible selection bias is that people voting for Trump might be systematically more ashamed of reporting this truthfully than people voting for Hillary. I know for a fact that this is the case in Denmark, where the share voting for the populist anti-immigrant party has for a long time had to have been corrected upwards to adjust for this effect. 
