In order to understand random sampling, you need to become familiar with a couple of basic statistical concepts.
Error - This is that "plus or minus X%" that you hear about. What it means is that you feel confident that your results have an error of no more than X%.
Confidence - This is how confident you feel about your error level. Expressed as a percentage, it is the same as saying if you were to conduct the survey multiple times, how often would you expect to get similar results.
These two concepts work together to determine how accurate your survey results are. For example, if you have 90% confidence with an error of 4%, you are saying that if you were to conduct the same survey 100 times, the results would be within +/- 4% of the fist time you ran the survey 90 times out of 100.
If you are not sure what sort of error you can tolerate and what level of confidence you need, a good rule of thumb is to aim for 95% confidence with a 5% error level.
Error is also referred to as the "confidence interval" and Confidence is also known as "Confidence Level." In order to avoid confusion, these concepts will simply be referred to as "Error" and "Confidence".
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