There are a lot of public opinion surveys right now, especially with all that is surrounding President Trump and the upcoming elections in two weeks.
How a sample is chosen and how a question is asked determine if a survey result will truly be providing projectable, statistically valid data.
A random sample means that everyone in a given population has an equal chance of being selected. Anything that alters this concept biases the results and makes it unlikely that a survey can be an accurate prediction of a given population.
If you sample everyone who is registered to vote—this will include a lot of people who will not vote.
If you sample likely voters in a way that excludes certain elements of the population (such as neighborhoods with a lot of low-income or minority populations), it’s likely that the results will not be accurate for the entire population being sampled.
If you sample using a self-select method (such as a Survey Monkey or some other on-line survey or a mail survey) it will exclude people who simply have no interest in responding.
As part of the survey methodology, the survey company will want to have a screening question, (assuming they are using a list of registered voters) such as:
“Are you planning to vote in the next election in your community?”
Surveys are a snapshot of the moment. If a major event happens that is widely reported in the news a few days before the polling is done, that will definitely affect the results.
It is also important to check, if you can, to see who is conducting the survey to see if it is a legitimate survey company and not simply one that is financed by a particular political party or promoting something,