Reply #9: Actually it's pure mathematics. [View All]

[Lucky2017]

When you call for example 800 people to ask for their political preference, there's always a (very very small) possibility that you end up with 800 republicans or 800 democrats or more realistic that the sample is not an exact replication of the situation that is examined. There no way to prevent that because if you want to take a good poll, you've got to have a random sample of the population. So, there's always a chance the sample and the results of the poll won't be a good representation of the reality.

The margin of error can be mathematically calculated. It is dependant on the size of the sample (if you poll almost every American, you will always have a good representation of the country and the margin of error will be almost zero, if you ask only 2 people for their presidential preference that can't be a good representation of the country and the margin of error will be very big).

An example: If a poll shows Obama with 52% en Mccain with 42% and the margin of error is 3%, this means that 95% of the times a poll like this one is taken the real situation will differ less than 3% from the poll. So there is a 95% chance that obama's real support is between 55% en 49% and McCain's between 39% an 45%. There's always a little chance (5%) that te real situation differs more than 5% from the poll's results.

I hope this explains it a bit. My excuses if my English is rather poor.