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As we approach the presidential election, polling statistics seem to be referenced more than ever.

I've never had the opportunity to be included in polls.

This leads me to wonder;

  • Who is polled?

How does a reputable polling agency, like Gallup, select people to poll for the US general election?

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    This question is impossibly broad to me without some research and refinement. If you go to 270 To Win or RCP you can easily find out who are the various organizations that do polling. From each one of those orgs you can find a lot of details about them and their methodology. – Brian Z Jun 18 at 1:48
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    And that's just elections polling... Public opinion polling in general is even broader, but Wikipedia has an article about the different general methodologies. – Brian Z Jun 18 at 1:51
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    I've edited to make this more focused. I now asks only about us general elections and the sampling methods used by pollsters. I think this is now answerable. – James K Jun 18 at 11:44
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It really depends on the pollster. Generally, a random sample is taken of the population, using methods such as random telephone number generation, and this sample is then weighted to correct for demographic size and sampling issues.

Gallup publishes information about how it samples the population for its weekly U.S. Poll. In this case, the sampling is done by randomly generating landline and cellphone numbers, and then randomly selecting a person from each household it rang. After results have been collated, the results are weighted, to correct for "unequal selection probability, nonresponse, and double coverage of landline and cellphone users", as well as to ensure that the demographics of the sampled population match the demographics of US population as a whole.

Some pollsters use online sampling instead of phone interviews. YouGov, for instance, uses 'Active Sampling', which uses data held about people signed up to their website to ensure that they have a ready-weighted sample of the population to poll. This then undergoes further weighting after results are collected.

Nate Silver has written previously about this, critiquing several sampling methods used by polling companies. Particularly, he mentions Rasmussen, which only conducts phone calls during certain time frames:

Typically, calls are placed from 5 pm to 9 pm local time during the week. Saturday calls are made from 11 am to 6 pm local time and Sunday calls from 1 pm to 9 pm local time.

Source - Rasmussen Reports

He also mentions other flaws:

They do not call phone numbers back, as most other pollsters do, in the event they don’t get an answer the first time. They don’t call cellphones — only landlines. And they speak to the first person they get on the line if they speak to anybody at all; other polling firms use carefully-designed procedures to randomize the selection of respondent within the household (a typical mechanism is something like asking that the adult with the next birthday come to the phone).

So you can see, the method of sampling depends greatly on the pollster, and therefore it is important to check the methodology of specific polls in order to contextualise their findings.

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