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→Nonresponse bias: HTTP --> HTTPS #3, replaced: http://abcnews.go.com → https://abcnews.go.com
===Nonresponse bias===
===Nonresponse bias===
Since some people do not answer calls from strangers, or refuse to answer the poll, poll samples may not be representative samples from a population. Because of this [[selection bias]], the characteristics of those who agree to be interviewed may be markedly different from those who decline. That is, the actual sample is a biased version of the universe the pollster wants to analyze. In these cases, bias introduces new errors, one way or the other, that are in addition to errors caused by sample size. Error due to bias does not become smaller with larger sample sizes. If the people who refuse to answer, or are never reached, have the same characteristics as the people who do answer, the final results will be unbiased. If the people who do not answer have different opinions then there is bias in the results. In terms of election polls, studies suggest that bias effects are small, but each polling firm has its own formulas on how to adjust weights to minimize selection bias.[http://abcnews.go.com/images/pdf/responserates.pdf]
Since some people do not answer calls from strangers, or refuse to answer the poll, poll samples may not be representative samples from a population. Because of this [[selection bias]], the characteristics of those who agree to be interviewed may be markedly different from those who decline. That is, the actual sample is a biased version of the universe the pollster wants to analyze. In these cases, bias introduces new errors, one way or the other, that are in addition to errors caused by sample size. Error due to bias does not become smaller with larger sample sizes. If the people who refuse to answer, or are never reached, have the same characteristics as the people who do answer, the final results will be unbiased. If the people who do not answer have different opinions then there is bias in the results. In terms of election polls, studies suggest that bias effects are small, but each polling firm has its own formulas on how to adjust weights to minimize selection bias.[https://abcnews.go.com/images/pdf/responserates.pdf]
===Response bias===
===Response bias===