About a third of those who say they will vote never show up, and some of them change their minds even if they do show up. That promise may not be fulfilled. This is particularly true of young people, who only turn out at about a 30 percent rate, even in a presidential year. Pollsters have to weight the sample based on these predictive assumptions. In many polls, likely voters may be a sub-sample, and predictions about their behavior will have a wider margin of error.
Samples come in various flavors.
Rather, there must be systematic random selection to ensure that each individual is chosen entirely by chance. This is the juncture where polls sometimes go wrong. Without randomized design, there can be no dependable statistical analysis. Non-randomness undermines the whole effort. No matter the appearance of figures and impressive formulas, a poll result without random design is a castle built on sand, and you might as well stick with hunches.